Migration of documentation to Nextcloud documentation

[bigcat88]

Task: Migrate and Refactor AppAPI Documentation for Nextcloud

Context:
We cannot make significant updates to our current AppAPI documentation because the more we update it now, the more we’ll have to redo later. Additionally, much of the old documentation was written before AppAPI became a built-in application. At that time, the documentation was simpler, as we assumed users knew why they were installing AppAPI and how to configure it.

Now that AppAPI is a built-in application, the documentation requirements have changed, necessitating this migration and refactor to better suit the new user base.

We need to migrate the current documentation from AppAPI's GitHub Pages to the official Nextcloud documentation, located here, and split it into two sections:

1. Admin Documentation: Content related to administration should go to the Nextcloud Admin Docs.
2. Development Documentation: Content related to development should be moved to the development manual, specifically the section on developing ExApps.

Each section should also have its own FAQ, which we'll review and complete later.

As part of this migration, the main tasks include:

1. Reread the entire documentation and remove outdated content.
2. Divide it into two parts: Admin and Development.
3. Rework the Table of Contents for each part to make it easier to find relevant sections and to reference from the AppAPI UI.
4. Add missing sections such as a detailed FAQ and more configuration examples where necessary.
5. Submit it for review, then merge it into the main Nextcloud documentation.

Remember, the documentation doesn't need to be perfect in this iteration; it just needs to be split and structured.
This is for Nextcloud version 30+ only, and it's important to reflect that AppAPI is now an embedded application.

Even draft PRs are acceptable, as long as we have a foundation to move forward.

[DaphneMuller]

Clarification on what is outdated content: a lot of content was written before AppAPI was a shipped app. Content that makes no longer sense due to this change, should be removed.