Tidy up references in documentation

[rwb27]

There are now more conceptual documentation pages, which means we should probably be linking less frequently to the Web of Things page, and more often to pages that describe the same concepts from a LabThings perspective. It would be good to finish adding these conceptual docs, then do a big search of references to the WoT page that should point elsewhere.

[rwb27]

I've searched through and removed the last few unnecessary references. This issue should be closed by #195.