Add `index page` section to documentation (#331)

* Add `index page` section to documentation

The current documentation does not describe how to create a index page.
I initially found this confusing, because I expected an index page to be
a **page** rather than a section.  Thus, I tried to access the page
content with `{{ page.content }}` and was very frustrated when I could
not.

This addition clarifies that the index page is **always** a section,
even if it does not have any sub-pages.  This should also help people
who intend to use Gutenberg to build stand-alone webpages, rather than
blogs.

docs/content/documentation/templates/overview.md

@@ -10,11 +10,7 @@ As this documentation will only talk about how templates work in Gutenberg, plea
 the [Tera template documentation](https://tera.netlify.com/docs/templates/) if you want
 to learn more about it first.
 
-All templates live in the `templates` directory and built-in or themes templates can
-be overriden by creating a template with same name in the correct path. For example,
-you can override the RSS template by creating a `templates/rss.xml` file.
-
-If you are not sure what variables are available in a template, you can just stick `{{ __tera_context }}` in it
+All templates live in the `templates` directory.  If you are not sure what variables are available in a template, you can just stick `{{ __tera_context }}` in it
 to print the whole context.
 
 A few variables are available on all templates minus RSS and sitemap:
@@ -23,6 +19,41 @@ A few variables are available on all templates minus RSS and sitemap:
 - `current_path`: the path (full URL without the `base_url`) of the current page, never starting with a `/`
 - `current_url`: the full URL for that page
 
+## Standard Templates
+By default, Gutenberg will look for three templates: `index.html`, which is applied 
+to the site homepage; `section.html`, which is applied to all sections (any HTML 
+page generated by creating a directory within your `content` directory); and 
+`page.html`, which is applied to all pages (any HTML page generated by creating a 
+`.md` file within your `content` directory).
+
+The homepage is always a section (regardless of whether it contains other pages).
+Thus, the `index.html` and `section.html` templates both have access to the 
+section variables.  The `page.html` template has access to the page variables.
+The page and section variables are described in more detail in the next section of this documentation.
+
+## Built-in Templates
+Gutenberg comes with three built-in templates: `rss.xml`, `sitemap.xml`, and 
+`robots.txt` (each described in their own section of this documentation).
+Additionally, themes can add their own templates, which will be applied if not 
+overridden.  You can override built-in or theme templates by creating a template with
+same name in the correct path. For example, you can override the RSS template by
+creating a `templates/rss.xml` file.
+
+## Custom Templates
+In addition to the standard `index.html`, `section.html`, and `page.html` templates,
+you may also create custom templates by creating a `.html` file in the `templates`
+directory.  These custom templates will not be used by default.  Instead, the custom template will _only_ be used if you apply it by setting the `template` front-matter variable to the path for that template (or if you `include` it in another template that is applied).  For example, if you created a custom template for your site's About page called `about.html`, you could apply it to your `about.md` page by including the following front matter in your `about.md` page:
+
+```md
++++
+title = "About Us"
+template = "about.html"
++++
+```
+
+Custom templates are not required to live at the root of your `templates` directory.
+For example, `product_pages/with_pictures.html` is a valid template.
+
 ## Built-in filters
 Gutenberg adds a few filters, in addition of the ones already present in Tera.