Code monitoring: update docs for webhooks and repo-aware monitors

Created by: camdencheek

This updates some documentation associated with the release of repo-aware monitors and webhooks.

Stacked on https://github.com/sourcegraph/sourcegraph/pull/37037

Test plan

Checked locally that it looks right.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.