review options for accessibility linting (versus documentation)

[cfm]

Require WCAG compliance along with other linting enforced automatically in CI. Here's an overview of how to do so and the W3C's list of such tools, of which Pa11y and AccessLint look especially promising.

Originally posted by @cfm in https://github.com/freedomofpress/securedrop/issues/5743#issuecomment-861867074