Add documentation for Mattermost AI Copilot Plugin

[azigler]

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This issue has been created following a conversation with @cwarnermm. See #7005 for umbrella issue.

Description

As part of our continuous effort to improve Mattermost documentation and make it more comprehensive for all user groups, we propose the addition of new documentation pages specifically dedicated to admin and user configuration for bundled, Mattermost-supported plugins. As the initial objective, we will do this for the new AI Copilot documentation to set up the new page flow.

Objective

Create new doc page(s) for the AI Copilot plugin. This page will guide different user groups to appropriate resources:

• Developers will be redirected to developers.mattermost.com for building custom extensions.
• Admins will be navigated to a new subpage under "Use Mattermost" titled "Enable and Configure Plugins", which will include documentation for each packaged plugin, including setup, configuration, and use cases.

Action Items

• [ ] Draft the initial content for the AI Copilot plugin documentation, including admin setup, configuration, developer customization options, and end user examples.

Please feel free to provide feedback or suggest any amendments to this plan.