Docs: Document RPC prefixes

The Rest API served by Octez on the RPC port is described by several OpenAPI specifications.

The full RPC served by a node is a union of:

• openapi.json
• For each active protocol:
  • $PROTOCOL-openapi.json (served under the prefix: /chains/<chain-id>/blocks/<block-id>)
  • $PROTOCOL-mempool-openapi.json (served under the prefix: /chains/<chain-id>/mempool)

This is not documented anywhere: we should do that.