[DOCS]: Write documentation for Roles, and Permissions component

Description

This issue is for rewriting the Roles and Permissions documentation during the Mautic migration. The goal is to make the content clearer, better structured, and aligned with the new documentation standards.

What is inaccurate, missing, or confusing
• The current explanation of permission bits lacks clarity and proper examples.
• The permission notation (plugin:bundle:level:permission) is not introduced clearly.
• Steps for creating custom permissions are incomplete and need clearer breakdowns.
• Helper methods (standard, extended, manage permissions) need better explanation.
• Some sections are unstructured and difficult for new developers to follow.
• Formatting and terminology are inconsistent with the new documentation style.

Relevant Documentation URL

https://developer.mautic.org/#how-permissions-work

Are you willing to help us fix this by making a pull request?

Yes

Suggestions (Optional)

No response

Screenshots or Screen Recordings (Optional)

No response

Start with the current Roles and Permissions page at https://developer.mautic.org/#how-permissions-work and locate its source in the developer-documentation-new repository. Review the permission-bit examples, permission notation, custom-permission steps, and helper-method explanations against the new documentation standards. Done means the content is clearer, consistently formatted, complete, and easier for new developers to follow.