Added a configuration option to determine whether the sort order is case-sensitive.
The default is case-insensitive.

To test:
- open `/just-the-docs/docs/utilities/` in the browser,
  and check that the navigation links in `Utilities` are sorted alphabetically;
- in `docs/utilities/layout.md', change the preamble to `title: layout`,
  and check that the  links in `Utilities` are still sorted alphabetically;
- add `nav_sort: case_sensitive` in the configuration file,
  and check that the link to `layout` is now listed last under `Utilities`.

When `nav_order` is omitted, the order of nodes at each menu level (and in the auto-generated TOC) is alphabetical by `title`, instead of random.
Any nodes with a specified `nav_order` precede all nodes at that level where it is omitted.
Note that `nav_order` fields must have a uniform site-ide type: integers and strings cannot be mixed, otherwise Jekyll reports errors.
The implementation filters the ordered and unordered pages from `site.html_pages`, sorts them separately, and concatenates the resulting arrays.

This reverts commit 0cfc0f03.

This reverts commit 8ac545a5.

This reverts commit c282e4b0.

This reverts commit 5d58a663.

This reverts commit 89f79138.

This reverts commit 209ac7be.

Restrict navigation to display only direct children.

This reverts commit 8ac545a5.

This reverts commit c282e4b0.

Separating new PR from #188, since indpendent.

The current default is that when a grandparent is selected, the links to its
children *and all its grandchildren* are displayed in the navigation menu.
Setting `grandchildren_branch: true` in `_config.yml` delays the display of links
to grandchildren until their parent is selected.

A `navigation-list-child-list` is not displayed unless the enclosing `navigation-list-item` is `active`, so it needs to be generated only when `page.url == node.url or page.parent == node.title or page.grand_parent == node.title`.

- Trimmed whitespace between html elements
  This reduced one of my files from 850KB to 115KB
- Do not sort the whole list of pages on every iteration
  This made build 3x faster