Add documentation for profile selectors

[eleftherias]

Summary

Ref #4018

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Outline how the changes were tested, including steps to reproduce and any relevant configurations.
Attach screenshots if helpful.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[coveralls]

coverage: 53.891% (-0.008%) from 53.899%
when pulling c3ba16b on eleftherias:4018-selectors-docs
into 98c9331 on stacklok:main.

[mesembria]

question: Do we want to explicitly call out that wildcards and partials are not supported, or do you feel this is sufficient?

[eleftherias]

Any kind of wildcard support would need to be part of CEL, so I've left it out of our docs