Close #686

Added and clarified a few things that would probably have saved me from
raising getzola/zola#691.

Close #586

Closes #636

Closes #533

Fixes https://github.com/Keats/gutenberg/issues/510. Well, should help with it.

Closes #485

Local CSV/TOML/JSON loading Tera function

Close #446
Close #260
Close #478
Close #284
Close #480

This allow to use base_url in robots.txt, to reference a sitemap for
example.

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 :)

Fix #307

* 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.

Closes #257