ci: Use lychee for linkchecking

[kratsg]

Pull Request Description

As requested in #2675 - trying out lychee instead for linkchecking.

• Use lychee for more robust linkchecking than Sphinx's linkcheck

pyhf/docs/Makefile

Lines 197 to 202 in 26962f5

	.PHONY: linkcheck
	linkcheck:
	$(SPHINXBUILD) -b linkcheck $(ALLSPHINXOPTS) $(BUILDDIR)/linkcheck
	@echo
	@echo "Link check complete; look for any errors in the above output " \
	"or in $(BUILDDIR)/linkcheck/output.txt."

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Use lychee for more robust linkchecking than Sphnix's linkcheck.
   - Use https://github.com/lycheeverse/lychee-action over local install.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.24%. Comparing base (26962f5) to head (ece766c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2676   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files          65       65           
  Lines        4220     4220           
  Branches      464      464           
=======================================
  Hits         4146     4146           
  Misses         45       45           
  Partials       29       29           

Flag	Coverage Δ
contrib	98.12% <ø> (ø)
doctest	98.24% <ø> (ø)
unittests-3.10	96.44% <ø> (ø)
unittests-3.11	96.44% <ø> (ø)
unittests-3.12	96.44% <ø> (ø)
unittests-3.13	96.44% <ø> (ø)
unittests-3.9	96.49% <ø> (ø)

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[matthewfeickert]

This looks good — thanks @kratsg! — but I'm confused what is going on with Codecov. :? How can changing how the linkchecks work affect the coverage? https://app.codecov.io/gh/scikit-hep/pyhf/pull/2676/indirect-changes

That doesn't make sense.

[matthewfeickert]

Okay, now it is passing. Weird. :? We might need to figure out if there's something up with how Codecov reports are getting uploaded.