Provide WoT "well known" Discovery endpoint describing Ditto WoT Directory

[thjaeckle]

Add a new endpoint to Ditto:

GET /.well-known/wot

Under which a Thing Description is served which describes the TDD (Thing Description Directory) - or simply Directory.

See also explainer: https://github.com/w3c/wot-discovery/blob/main/explainer/Explainer.md