Treat docs warnings as errors

[davidfischer]

This PR builds on top of #5819. Start reviewing at commit 19ec28a.

• Using the makefile in docs/ will treat warnings as errors during docs generation
• The docs tests tox -e docs will treat warnings as errors during testing and CI
• Fixes remaining docs warnings

[stsewd]

Can you try removing these too?

https://github.com/rtfd/readthedocs.org/blob/81b8cf19dd9c4580ab3d90378222923a1b83e248/docs/.rstcheck.cfg#L4

[davidfischer]

Can you try removing these too?

I was able to remove (Duplicate implicit target name: ".*") and fix the lingering issues but (Hyperlink target ".*" is not referenced) is needed until https://github.com/myint/rstcheck/issues/19 is fixed.

For example, here's one error it reports that looks erroneous:

./api/v2.rst:174: (INFO/1) Hyperlink target "api-version-detail" is not referenced.

Here's that line in the rst file but the hyperlink target is referenced just a few lines earlier.

Only a couple of the errors of this type are erroneous and I was able to fix the others.

[stsewd]

Finally! really close to a full lint :)