Initial refactor

[rhiaro]

This PR only touches structure. There are still (editorial) content related issues that will be addressed in a future PR if this starting point is okay.

• Moved the URIs/URNs/URLs section into Terminology - for the time being at least. This needs more work, content-wise, and depending on that may get moved again. ([URIs, URLs, and URNs] Section doesn't conclude with a precise declaration of what a DID is. #155, [URIs, URLs, and URNs] "DIDs have a foundation in URLs" is confusing because DIDs aren't URLs #156, Updates to URIs, URLs, and URNs section #159)
• Made a top level DID Methods section, which now contains DID Method Schema and DID Operations ([Extensibility] references the formal concept of the Decentralized Identifiers Data Model but there is nothing in the document with this formal name/concept #151).
  • TODO: there's language about DID Methods in Persistence, in the DIDs section, which might need adjusting out into the DID Methods section, but it's pretty tightly bound up so leaving that for a future PR.
• Bumped Purpose and Simple Examples up a level, which based on the hs they used to be anyway, so I think this might have been a section nesting accident. ([PURPOSE OF THE SPECIFICATION] Is this draft specification trying to address too many topics when there should be more than 1 spec #121, [Introduction] Is the Purpose of the Draft DID spec clear and correct? Is there a defined Target Audience for the document... #157)
• Marked non-normative sections (Make clear which sections of the spec are normative #138)
  • TODO: there is normative language in the Security and Privacy consideration sections which needs moving to normative parts of the spec (will make an issue for this if there isn't one already).

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[msporny]

Nothing controversial here, just a refactor. Merging.