docs: add direct editing developer documentation

Document the direct editing API: editor registration (IEditor, creators,
templates), the OCS HTTP API, and the webview messaging convention used by
the mobile clients.

AI-assistant: Claude Code 2.1.160 (claude-opus-4-8)
Signed-off-by: Julius Knorr <jus@bitgrid.net>

Author: juliusknorr