Add automatic accessibility testing

[trallard]

We want to make sure that our handbook as our website is accessible to all users and adheres to WCAG (Web Content Accessibility Guidelines) guidelines.

With the recent upgrade to the theme, we need to ensure this is accessible to all.

What needs doing?

• Add Github action to build the site
• Extend the action to test the site using pa11y

📖 Resources

• mentored-sprints.dev action
• mkdocs actions in marketplace
• pa11y docs
• pa11y ci