[DevDocs] Rework the API documentation

[mmattel]

The API documentation in owncloud.dev needs some urgent and intense love, it is when looking at it in a messy state.

We must have:

• A central entrypoint for all stuff that is API related
• A clear structure about the content, what to expect and properly written
• All API doc content must reside here (no pointing to oc10 dev docs etc)

Post revision, it of course needs regular maintenance.

@tbsbdr @micbar @butonic @fschade

[micbar]

This is too general. We need small and actionable tasks. Duplicate of #5117. Closing here.

[mmattel]

This is too general. We need small and actionable tasks.

That was exactly the reason adding this issue. The referenced issue describes content changes while this one describes structure.