Rework OCM-API diagrams, sync OpenAPI to draft

[MahdiBaghbani]

This makes the OCM-API spec easier to navigate and to implement from.
Three things landed here, all in service of the same goal: help implementers and users find the normative home for each concept.

• A navigation index and a signing direction index in the I-D, so the four
  orthogonal planes and the per-endpoint signing rules have a single pointer
  table.
• A reworked diagrams/ folder: 8 numbered diagrams plus a README.md,
  with a unified palette and a style contract.
• spec.yaml aligned to the current I-D draft.

The part that made me stop and think for some time is that the spec described the four planes (discovery, share, peer interaction, enforcement) only implicitly, and the diagrams did not share a vocabulary or palette.

An implementer reading IETF-OCM.md had no single place to land.
I think this is the right direction: keep the draft normative, add informative pointers and diagrams, and make the OpenAPI match the draft.

No new normative rules are introduced by the diagrams or the indexes

[glpatcern]

This is all very good, in particular the diagrams are excellent!
As discussed, some relatively minor changes on the yaml

[glpatcern]

Looks good! In particular the diagrams are now much more descriptive, I think some adjustments can still be done but it's a very good progress already.