Create draft keyboard navigation event content

[isabela-pf]

The long-awaited second event type is being prototyped! 🎉

This PR introduces some (very) draft resources for testing out an event for reviewing keyboard navigation and contributing a review document to identify issues. This could be run on a whole interface, on a PR (with permission), probably even on desktop apps.

While it doesn't directly fix all the issues, this lack of review knowledge in the broader community has meant that lots of work passes through review missing some foundational accessibility needs. The tests listed in review-template.md may be helpful for reviewers even outside of event contexts. 🌻

This PR:

• Creates a new directory workshop-resources/keyboard-navigation
• Adds a README.md to describe keyboard navigation events and index the resources
• Adds an agenda-template.md as an incomplete event agenda
• Adds a review-template.md to provide the main content and document participants will contribute to during the event

Phew. 😅 There's a lot in these three files, but the review-template.md is the only entirely new piece of content. The rest are based on the image description event resources.

[isabela-pf]

https://accessibility.18f.gov/keyboard/

[isabela-pf]

I'm going to merge this since I haven't gotten further feedback. I'm expecting many changes will need to be made as this event starts getting tested. Onwards!