Add docs for using feature flags when developing Minder

[evankanderson]

Summary

Adds documentation on how to use feature flags during development.

Fixes #3187

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

No tests, sorry!

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: 50.523% (+0.006%) from 50.517%
when pulling 681e86f on evankanderson:3187-allow-configuring-flags-in-helm-chart-1
into a4eabe5 on stacklok:main.