Add docs for provider auto-registration

[jhrozek]

Summary

Summarizes the current state in minder OSS documentation

Related: #3458

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

build and read the docs

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.

[jhrozek]

Once this is merged, I'll get the SaaS docs up - they will be a bit different since minder cloud combines the two operations (that's why I went ahead and implemented the CLI -all flag).