Add local link checking to read the docs

[Ldcabansay]

Is your feature request related to a problem? Please describe.
We currently only test for broken external links when doing link checks on read the docs. We should add local link checks so errors/warnings pop up when references to internal links are broken.

Describe the solution you'd like
make linkcheck should also check for broken internal links and give errors when found. Consider how link checks will relate to re-directs (if necessary).

Describe alternatives you've considered
We also can enforce an SOP that uses preferred way to refer to local links (ie only absolute paths, only certain types of referencing styles, etc)

Additional context
This is part of the documentation overhaul, should probably be completed before/during adding re-directs.

┆Issue is synchronized with this Jira Story
┆Issue Number: DOCK-1570
┆Sprint: Backlog
┆Issue Type: Story

[melainalegaspi]

@Richard-Hansen completed this

[coverbeck]

Fixed via dockstore/dockstore-documentation#85