Fixed typo from 'ad' to 'at'.

This commit expands the Netlify documentation page to include a
description of how to manually deploy a built Gutenberg site from the
command line.

This is extremely minor, but the initial `netlify.toml` example confused
me a bit because I didn't realize that it was using a *netlify*
variable; I thought I needed to replace something with a variable from
my own configuration.  I made a very minor edit to the docs to clarify.

Fix #307

Close #115

* 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.

Add details to the templates/RSS page to clarify that the `rss.xml`
page is only generated if the `generate_rss = true` variable is set
in the site's `config.toml` page.  This information is already present
in other parts of the documentation, but is not present in the RSS page,
which could be confusing.

- Fix some typos
- Some small changes to documentation to make it more clear
- Change the themes list from text to links