Add a guide for agentic access to GitHub via Beams - #69580

Add a guide to using Beams with Teleport-protected GitHub organizations
in `docs/pages/beams`. Unlike the database guide, this guide does not
lend itself to a quick, single-purpose demo, so the guide offloads the
work of setting up GitHub with Teleport to our pre-existing tutorial.
This guide provides a high-level overview of the flow for using Beams
with a Teleport-protected GitHub organization.

Related changes:
- Edit guide titles: Since the GitHub guide cannot be a demo guide, edit
  the database guide to make the titles of the two guides consistent.
- Also add an audit step to the database guide. The GitHub guide adds
  step to check audit events. Mirror this change in the database guide
  since it's a straightforward edit.