Remove pa11y infrastructure from this documentation

[choldgraf]

Our pa11y build is broken causing all of our CI/CD to fail. I spend an hour trying to figure out how it works and un-break it, but I cannot figure out why it is failing, nor was I able to successfully get a pa11y audit working locally.

Given that:

• We already test for some accessibility issues via Lighthouse
• We do not have team knowledge of the pa11y infrastructure setup
• That setup is now broken and can't easily be fixed
• I didn't get any objections to the proposal to remove pa11y here.

This is a PR to remove pa11y from our documentation builds. It is unfortunate to remove some extra checks on accessibility, but I hope that we can make up for this by ensuring our Lighthouse builds are solid, and I think the tradeoff is worth the extra maintenance and complexity penalty we are paying.

closes #720

[choldgraf]

OK we have two 👍 - I'll plan to merge this in the next day or two unless anybody objects and proposes and alternate path forward.

[choldgraf]

OK, merging this one in! I am happy to review any future PRs that further improve upon our accessibility infrastructure via Lighthouse, or that find a way to build in pa11y support in a way that is sustainable/maintainable for our team of maintainers.