ci: add pa11y soft check in GitHub Actions

[logans-stuff]

CI/CD accessibility check summary

I added a new non-blocking accessibility audit job to the GitHub Actions workflow.

What changed

• Kept the existing build job unchanged so mkdocs build --strict still validates the site as before
• Added a separate accessibility job that runs on every push and pull request
• Installed both the MkDocs Python dependencies and Node.js so pa11y-ci can run in GitHub Actions
• Built the site and served the generated site/ directory locally inside the GitHub Actions runner
• Ran pa11y-ci against the existing .pa11yci configuration in the repo root
• Marked the pa11y-ci step with continue-on-error: true so it behaves as a soft check
• Uploaded the pa11y results and local server log as workflow artifacts for troubleshooting
• Left the deploy job unchanged, so deployment behavior to GitHub Pages stays the same

Result

Accessibility issues now show up in CI as advisory feedback without blocking merges or deploys. This gives the team visibility into regressions while keeping the current release flow intact.

Docs update

• Updated ACCESSIBILITY.md to document that pa11y-ci now runs automatically in GitHub Actions as a soft check