v18, v17, Docs: Documentation for Octez v18.0~rc1 and Octez v17.3

Context

Update of the documentation for Octez v18.0~rc1

It includes:

• New release page version-18
• Update of the openapi specifications:
  • New rpc-openapi-rc
  • New specs for Oxford
  • Specs for Nairobi on v18
• Snapshot of the changelogs

It also includes changes for the release of Octez v17.3

Manually testing the MR

Read the generated documentation

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR