Enhancement: Add axe-core to testing for pages to enhance accessibility regression testing

[milofultz]

We use the axe linter at work all the time to implement regression testing, and I think using it in our test suite would be super helpful. I found a matcher for vitest, too.

A lot of work would need to be done to be able to even implement these tests, as I think we would need to bring our pages up to be at 0 failures when using the matchers, first (see codeforpdx/codepdx_website#55). But this would be huge for clamping down on pages that are already there and for pages where we've done the work to make them completely accessible.