Doc: reorganize and rename docs

- Move general docs to src/webengine/doc/src
- Remove a dedicated .qdocconf for Qt WebEngine Core,
  build its documentation as part of Qt WebEngine
- Describe all Qt WebEngine modules on the Qt WebEngine
  index and overview pages
- Add a wireframe drawing on the architecture

Change-Id: Ib4261d1a16bbaf313d0f858515184a96dab524a5
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>