Add docs for user management and invitations (#3837)

[rdimitrov]

Summary

Details:

• Add conceptual docs on projects, document how to create new projects, because I was there
• Add role requirements to how-to prereqs
• Clarify one role per project
• Rewrite adding users to describe invitations
• Undo excessive prettier formatting
• Update docs/docs/how-to/create_project.md
• Switch naming from Stacklok (vendor) to Minder (project) in documentation where I missed it.

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: 54.219%. remained the same
when pulling 7880cb4 on 3795-add-docs-for-user-management-and-invitations
into ac3241f on main.