PBM-1790 Document PBM user database roles

[radoslawszulgo]

This pull request enhances the documentation for Percona Backup for MongoDB by adding a comprehensive reference for the required database user roles. It introduces a new reference page that details each role's purpose, scope, and access, and links to this new resource from the authentication configuration guide. The navigation is also updated to include the new reference page.

Documentation improvements:

• Added a new reference page pbm-user-roles.md that explains all MongoDB roles required by PBM, including built-in and custom roles, their scopes, access types, and justifications for each.
• Updated the authentication configuration guide in configure-authentication.md to link to the new PBM user roles reference for further explanation of each required role.
• Updated the documentation navigation in mkdocs-base.yml to include the new PBM user roles reference page.

[Copilot]

Pull request overview

This PR adds a new documentation reference page describing the MongoDB roles required for Percona Backup for MongoDB (PBM), and links to it from the authentication configuration guide. It also updates the MkDocs navigation to include the new reference.

Changes:

• Added a new reference page documenting required PBM database user roles (readWrite, backup, clusterMonitor, restore, and custom pbmAnyAction).
• Linked the authentication configuration page to the new roles reference for additional explanation.
• Added the new reference page to the MkDocs navigation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
mkdocs-base.yml	Adds the new roles reference page to the “Reference” navigation section.
docs/reference/pbm-user-roles.md	Introduces a new reference page describing PBM-required MongoDB roles and their rationale.
docs/install/configure-authentication.md	Links readers to the new roles reference from the authentication setup steps.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.