Documentation fixes

[chcondon]

Description

corrected a broken link in template for sphinx doc, added link checker github action to test.yaml

Motivation and context

Will check for broken links in the future

Resolves #???

How has this been tested?

Built docs locally, credits no accessible from every page. Trying test.yaml right now

Change log

fixed broken link in sphinx-doc/_templates/page.html and added lychee broken link checker to github actions

Checklist:

• [x] I have reviewed the Contributor Guidelines.
• [x] I agree with the terms of the HOOMD-blue Contributor Agreement.
• [ ] My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.

[github-actions[bot]]

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