Replace `populate_next_and_previous_page` with `populate_siblings`

This commit removes the option to sort by order and also removes
`page.next` and `page.previous` variables.  Instead, pages can be sorted
by two methods `date` and `weight`.  The Tera `reverse` filter will
reverse either of those sorts, so the old `order` behavior can be
achieved by using the `reverse` filter with `weight`.

In place of the `previous`/`next` variables, this commit adds the
`page.earlier`/`page.later` variables (which are set when the page is
sorted by date) and the `page.heavier`/`page.lighter` variables (which
are set when the page is sorted by weight).  These variables have the
advantage of not having confusing semantics when the `reverse` filter is
used.

Fix typo in page.md doc

Fixed typo from 'ad' to 'at'.

Expand Netlify doc page

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

* Remove www.bharatkalluri.in from EXAMPLES.md

The footer and source code at <https://www.bharatkalluri.in/> state that
the site was made with Hugo, so it should be deleted as an example of a
Gutenberg site.

I considered also removing the link to https://adrien.is/, since the
link appears to be dead.  But I left it for now, since that could be a
temporary outage.

If you would like, I'd be happy to format EXAMPLES.md as a table, which
might help keep it looking clean as it gets more entries.

* Convert EXAMPLES.md to table

Add codesections.com to EXAMPLES.md

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.

Fix #307

Add an external link checker

Close #115

* 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.

Custom 404 page

Since we are using a builtin template, we can be assured to have the
target file in place. Middleware renamed since it is bound directly to
the concept of handling 404s.

This is a proposed successor to #296 (rewritten for actix-web, rather
than iron).

Users can now place custom error pages in static/error/<error_code>.html
e.g. static/error/404.html

If no custom page is found, a default plaintext describing the error is served.