This commit fixes 4 more broken links: the navigation menu item,
the reference-less link, and two other blog posts.

The `_navigation.yml` and several pages linked to
`/contributing/#evolution-process`, and we had an `<a>` tag with
`name="evolution-process"`, which is invalid in `htmlproofer`s eyes
because it's a link that does not have a reference.

Jekyll makes `id`s for every heading automatically, and we have
`/contributing/#swift-evolution` that we should link to instead.

The 404.html page renders with HTTP404 code, but that's valid. Hence, we
should ignore that page in `check_links.sh`

We have just one link without the `href` attribute, which is a button in
the Swift Evolutions browser. We know it's valid, and the case where `<a>`
tags won't have `href`s is rare, so we can safely ignore them in the
check.

In 0317303, I've added `htmlproofer` to
`Gemfile`, but did not commit the changes to `Gemfile.lock`. This commit
adds it back.