Add linkcheck to docs workflows

[matthewfeickert]

With the addition of user-defined build jobs in ReadTheDocs I noticed that one of their examples was to perform a check for broken links with sphinx's linkcheck.

I'm working on adding this both to the ReadTheDocs config and to the docs GHA workflow, but at the moment

$ cd docs
$ make linkcheck

is giving a failure

(           babel: line    3) broken    cli.html#pyhf-xml2json -

on

pyhf/docs/babel.rst

Line 5 in e7996e5

Many of these solutions involve extracting out the ``HistFactory`` workspace and then running `pyhf xml2json <cli.html#pyhf-xml2json>`_ which provides a single JSON workspace that can be loaded directly into ``pyhf``.

I'm not quite sure why, as this is a valid link once the source is built, but I think it might be a form of sphinx-doc/sphinx#9383.

I have this and other fixes on a branch named docs/use-read-the-docs-pre-build-job.

[matthewfeickert]

This can be solved as suggested in sphinx-doc/sphinx#7056 by simply adding the following to docs/conf.py:

linkcheck_ignore = ['cli.html#pyhf-xml2json']