Web plugin docs 11559

This PR adds docs for:

 - TOP_LINKS - how to add links to the top of the webclient.

 - Webclient plugin - how to display pages from an app within centre or right panels of webclient.

To test: I guess you need to follow the docs and see if the steps make sense and work as described.
This could be quite a bit of work (especially the plugin one) - maybe should be split between a couple of reviewers.