docs: consolidate developer documentation for the Activity API

Replaces the one-line stub with comprehensive inline documentation
covering all extension interfaces (IProvider, ISetting, IFilter),
event creation, rich object strings, activity merging, info.xml
registration, and the full REST API v2 reference with examples.

Links to the in-repo docs for additional detail.

AI-Assisted-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Anna Larch <anna@nextcloud.com>

Author: miaulalala