* 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

Closes #259

Closes #257

Dates must not be surrounded by quotes.

Closes #254

* Add ignored_content to the Config structure.
* Use the GlobSet crate to parse the glob patterns into a matcher, which
  is created once at program initialization. If there are no patterns in
  ignored_content, an empty globber is created, which excludes no files.
  This is consistent with the existing behaviour of Gutenberg, before
  this feature was added.
* Bail if there are any errors in the glob patterns.
* Add a call to the globber in page.rs to actually do the filtering.
* Update documentation.

A note on the Config structure
------------------------------
* I had to remove the PartialEq derive from the Config structure as it
  does not work for the GlobSet type. No harm is done, Config does not
  need to be PartialEq anyway, since there is no need to sort Configs.
* The implementation follows the pattern of the existing config settings
  in that it uses an Option<...>. This would appear unnecessary, in that
  an empty vec could be used as the default, but it appears to be needed
  by the TOML parsing. A better approach would be to use a separate
  SerializableConfig and map to/from a Config struct. This would also
  allow the elimination of most, if not all, of the other Options in
  the Config structure, but that ought to be another PR.