Add html-proofer link check for 4.x docs

[DomGarguilo]

This PR adds an internal link checker to the CI to make sure we are actively fixing any dead/broken links.

The config/setup for things to run in the CI was added in 980113f

Tooling added:

• html-proofer added to the Gemfile
• Rakefile: defines the task (named links here) which opens the built site (from _site/), checks all internal links against the files that actually exist, for now skips checks for everythign outside of the docs/4.x dir, prints errors if there are any

Here are some resources I used to add this feature:

• https://github.com/gjtorikian/html-proofer
• https://jekyllrb.com/docs/continuous-integration/travis-ci/
• https://github.com/ruby/setup-ruby

[dlmarion]

I don't have any experience with this, but the changes look benign to me.

[DomGarguilo]

Marking this as ready for review now. Once #495 is merged the failing checks will pass.

This PR fixes ~50 errors from the tooling added.

An example of what the new CI check looks like when it fails can be seen at 231e026. Here is a snippet from that failing check:

* At ./_site/docs/4.x/administration/in-depth-install.html:936:

  internally linking to /docs/4.x/configuration/server-properties#tserver_port_search; the file exists, but the hash 'tserver_port_search' does not

* At ./_site/docs/4.x/administration/in-depth-install.html:937:

  internally linking to /docs/4.x/configuration/server-properties#replication_receipt_service_port; the file exists, but the hash 'replication_receipt_service_port' does not


HTML-Proofer found 16 failures!

That commit makes these issues fail the build instead of warn so it now catches the broken links that will be fixed as of the changes in #495

Update: the build passes after merging. 92132cd

[ctubbsii]

Very cool. I just have a few suggestions to improve the wording around some of the broken links that were fixed.