Discoverable destinations

[matatk]

Changes made:

• Rename the destinations explainer to 'discoverable-destinations.md.'
• Rename the proposal within the explainer.
• Rename the proposal within the destinations respec document.
• Rename the document (within the intro section) from 'specification' to 'best practices document'.

Notes/questions for the group:

• The shortname for the destinations respec (best practices) doc is currently set to 'adapt-destinations' - do we want that?
• The directory name within the repo is 'destinations' - do we want that? Should we harmonise them?
• There are still a few occurrences of 'well-known' in the destinations best practice doc that need to be replaced by something like 'frequently accessed' or similar (or not; we could leave them for now).

Explainer preview: https://github.com/w3c/adapt/tree/discoverable-destinations/explainers/discoverable-destinations.md

Best practices doc preview: https://raw.githack.com/w3c/adapt/discoverable-destinations/destinations/index.html