add a target for running pa11y

[akcano]

This adds an option to run interactive CLI-based accessibility checks with 'make pa11y'. The GitHub workflow will be added separately in canonical/documentation-workflows#12.

This closes issue #28 on GitHub.

Links:

• https://pa11y.org
• https://accessibility.civicactions.com/posts/automated-accessibility-testing-leveraging-github-actions-and-pa11y-ci-with-axe

[akcano]

Looks good to me overall, a few small comments.

Would you mind splitting the commit in two - one for adding the action, one for fixing findings?

Sure, will do, after addressing the concerns you and @s-makin raised.