Also fix tests

Close #367, #372

I love the new support for custom taxonomies! I got a little thrown off when updating my site, though, as the docs didn't specify how the page would expose the taxonomies to the template. Wasn't too hard to figure it out, but I figured I'd save the next person who tries it the effort :)

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