[doc] Add some link to the middleware documentation
[doc] Rework a bit the doc of middleware

Remove 'available_methods', the information is available and up2date in
the matrix.
Add 'configuration specificities' to tell about keyword accepted by
add_stream.
Sync with reality :)
[camera] create new scene for each camera

- add scenes as underlay (not shown)
- add basics after creating scenes
- set viewport to camera
- do not create scene for external robot
- update objects' pose/orientation for each scene

fix #371
[test] update video_camera pgm images
[ros] test request manager init (log warning)