Added .liquid suffix to included scss files

(cherry picked from commit 7fb79c2018b979fdf917f427ca5d3f2451f3265d)

_includes/css/just-the-docs.scss → _includes/css/just-the-docs.scss.liquid

 @import "./support/support";
 @import "./color_schemes/{{ include.color_scheme }}";
 @import "./modules";
-{% include css/custom.scss %}
+{% include css/custom.scss.liquid %}

assets/css/just-the-docs-dark.scss

 ---
 ---
-{% include css/just-the-docs.scss color_scheme="dark" %}
\ No newline at end of file
+{% include css/just-the-docs.scss.liquid color_scheme="dark" %}
\ No newline at end of file

assets/css/just-the-docs-light.scss

 ---
 ---
-{% include css/just-the-docs.scss color_scheme="light" %}
\ No newline at end of file
+{% include css/just-the-docs.scss.liquid color_scheme="light" %}
\ No newline at end of file