Rewrite of the spec

[pietercolpaert]

This PR:

• Splits off discovery in its own spec (related: ViewDescription and multiple descriptions #96 dcat:DataService vs. dcat:Distribution #100)
• Reviews the non-normative introduction/overview section (related: Add an introduction #94 )
• Rewrites the spec from the perspective of the client (related: Writing the spec from the perspective of a client vs. a server #112 )
• Adds text for clients traversing (related Proof-reading session on traversing relations #104)
• Splits off compatibility as a draft spec: it is to be reviewed
• Splits off imports as a draft spec: it is to be reviewed

This indirectly also closes #92 (comment)

To be discussed:

• The discovery specification should at this point probably also be moved to a draft, as this has not yet been fully cleared out yet.
• The traversal spec does not yet reach the level of quality that we would like. People who are specialized in client-side processing should have a look.