Edit the Mattermost guide

[ptgott]

Fixes #14577

Update the guide and make it easier to use:

• Give this guide the structure of the Slack guide, adding stepped headings and a section on configuring Access Requests, to make this guide easier to follow step by step.
• Add the initial Access Request RBAC setup as a partial
• Indicate that this has been tested with Mattermost v7.0.1
• Add clarity tweaks throughout
• Update the instructions for editing the plugin configuration. The configuration fields have changed since guide was written.

[ptgott]

@zmb3 @r0mant @xinding33 Would one of you have time to review this? Thanks!

[github-actions[bot]]

@ptgott See the table below for backport results.

Branch	Result
branch/v10	Create PR
branch/v8	Failed
branch/v9	Failed