use toc.html from the amazing https://github.com/allejo/jekyll-toc

_includes/page_toc.html

+{% capture tocWorkspace %}
+    {% comment %}
+        Copyright (c) 2017 Vladimir "allejo" Jimenez
+
+        Permission is hereby granted, free of charge, to any person
+        obtaining a copy of this software and associated documentation
+        files (the "Software"), to deal in the Software without
+        restriction, including without limitation the rights to use,
+        copy, modify, merge, publish, distribute, sublicense, and/or sell
+        copies of the Software, and to permit persons to whom the
+        Software is furnished to do so, subject to the following
+        conditions:
+
+        The above copyright notice and this permission notice shall be
+        included in all copies or substantial portions of the Software.
+
+        THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+        EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+        OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+        NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+        HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+        WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+        FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+        OTHER DEALINGS IN THE SOFTWARE.
+    {% endcomment %}
+    {% comment %}
+        Version 1.0.12
+          https://github.com/allejo/jekyll-toc
+
+        "...like all things liquid - where there's a will, and ~36 hours to spare, there's usually a/some way" ~jaybe
+
+        Usage:
+            {% include toc.html html=content sanitize=true class="inline_toc" id="my_toc" h_min=2 h_max=3 %}
+
+        Parameters:
+            * html         (string) - the HTML of compiled markdown generated by kramdown in Jekyll
+
+        Optional Parameters:
+            * sanitize     (bool)   : false  - when set to true, the headers will be stripped of any HTML in the TOC
+            * class        (string) :   ''   - a CSS class assigned to the TOC
+            * id           (string) :   ''   - an ID to assigned to the TOC
+            * h_min        (int)    :   1    - the minimum TOC header level to use; any header lower than this value will be ignored
+            * h_max        (int)    :   6    - the maximum TOC header level to use; any header greater than this value will be ignored
+            * ordered      (bool)   : false  - when set to true, an ordered list will be outputted instead of an unordered list
+            * item_class   (string) :   ''   - add custom class(es) for each list item; has support for '%level%' placeholder, which is the current heading level
+            * baseurl      (string) :   ''   - add a base url to the TOC links for when your TOC is on another page than the actual content
+            * anchor_class (string) :   ''   - add custom class(es) for each anchor element
+            * skipNoIDs    (bool)   : false  - skip headers that do not have an `id` attribute
+
+        Output:
+            An ordered or unordered list representing the table of contents of a markdown block. This snippet will only
+            generate the table of contents and will NOT output the markdown given to it
+    {% endcomment %}
+
+    {% capture my_toc %}{% endcapture %}
+    {% assign orderedList = include.ordered | default: false %}
+    {% assign skipNoIDs = include.skipNoIDs | default: false %}
+    {% assign minHeader = include.h_min | default: 1 %}
+    {% assign maxHeader = include.h_max | default: 6 %}
+    {% assign nodes = include.html | split: '<h' %}
+    {% assign firstHeader = true %}
+
+    {% capture listModifier %}{% if orderedList %}1.{% else %}-{% endif %}{% endcapture %}
+
+    {% for node in nodes %}
+        {% if node == "" %}
+            {% continue %}
+        {% endif %}
+
+        {% if skipNoIDs == true %}
+            {% unless node contains "id=" %}
+                {% continue %}
+            {% endunless %}
+        {% endif %}
+
+        {% assign headerLevel = node | replace: '"', '' | slice: 0, 1 | times: 1 %}
+
+        {% if headerLevel < minHeader or headerLevel > maxHeader %}
+            {% continue %}
+        {% endif %}
+
+        {% assign _workspace = node | split: '</h' %}
+
+        {% assign _idWorkspace = _workspace[0] | split: 'id="' %}
+        {% assign _idWorkspace = _idWorkspace[1] | split: '"' %}
+        {% assign html_id = _idWorkspace[0] %}
+
+        {% assign _classWorkspace = _workspace[0] | split: 'class="' %}
+        {% assign _classWorkspace = _classWorkspace[1] | split: '"' %}
+        {% assign html_class = _classWorkspace[0] %}
+
+        {% if html_class contains "no_toc" %}
+            {% continue %}
+        {% endif %}
+
+        {% if firstHeader %}
+            {% assign firstHeader = false %}
+            {% assign minHeader = headerLevel %}
+        {% endif %}
+
+        {% capture _hAttrToStrip %}{{ _workspace[0] | split: '>' | first }}>{% endcapture %}
+        {% assign header = _workspace[0] | replace: _hAttrToStrip, '' %}
+
+        {% assign indentAmount = headerLevel | minus: minHeader %}
+        {% assign space = '' %}
+        {% for i in (1..indentAmount) %}
+            {% assign space = space | prepend: '    ' %}
+        {% endfor %}
+
+        {% if include.item_class and include.item_class != blank %}
+            {% capture listItemClass %}{:.{{ include.item_class | replace: '%level%', headerLevel }}}{% endcapture %}
+        {% endif %}
+
+        {% capture anchor_body %}{% if include.sanitize %}{{ header | strip_html }}{% else %}{{ header }}{% endif %}{% endcapture %}
+        {% capture anchor_body %}{{ anchor_body | replace: "|", "\|" }}{% endcapture %}
+
+        {% if html_id %}
+            {% capture list_item %}[{{ anchor_body }}]({% if include.baseurl %}{{ include.baseurl }}{% endif %}#{{ html_id }}){% endcapture %}
+        {% else %}
+            {% capture list_item %}{{ anchor_body }}{% endcapture %}
+        {% endif %}
+
+        {% capture my_toc %}{{ my_toc }}
+{{ space }}{{ listModifier }} {{ listItemClass }} {{ list_item }}{% if include.anchor_class %}{:.{{ include.anchor_class }}}{% endif %}{% endcapture %}
+    {% endfor %}
+
+    {% if include.class and include.class != blank %}
+        {% capture my_toc %}{:.{{ include.class }}}
+{{ my_toc | lstrip }}{% endcapture %}
+    {% endif %}
+
+    {% if include.id %}
+        {% capture my_toc %}{: #{{ include.id }}}
+{{ my_toc | lstrip }}{% endcapture %}
+    {% endif %}
+{% endcapture %}{% assign tocWorkspace = '' %}{{ my_toc | markdownify | strip }}

_layouts/page.html

@@ -4,4 +4,6 @@ layout: default
 
 <h1 id="title-heading" class="page-title">{{ page.title }}</h1>
 
+{% include page_toc.html html=content %}
+
 {{ content }}