[Gateway] Document traffic source and destination selectors

[Noir01]

Summary

Adds Gateway policy selector documentation for Traffic sources and Traffic destinations, including supported policy types, selector values, API examples, and limitations.

Also adds brief context to related Gateway and learning path pages so administrators can understand how to scope policies by where traffic enters and exits Gateway.

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.