Revise netlify.toml example

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.

* Update comparison in README for Hugo 0.43

* 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.

Edit docs to clarify RSS process

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.

Add snacraft config

This allows the easy generation of snaps as requested in #227.

This commit does not add automatic generation and publication of snaps
to the Snap Store, but they can easily be generated for local/personal
use by

```
snapcraft
sudo snap install gutenberg_xxx.snap
```

Add directories with snapcraft artifacts to `.gitignore`.

Closes #277

Theme deployment

Obfuscate <!-- more --> in docs so that it doesn't get replaced

Fix #285

Fix documentation typos and minor changes:

Add Tinkering to EXAMPLES.md

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

Next version

Closes #275

Closes #272

Closes #228
Closes #229

Anchor tag for summary

Close #265

Add docs for Sass processing

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