Docs, v21: Documentation for Octez v21.0 rc1 and rc2

What

To be merged only on release candidate publish day !

Introduce documentation for Octez v21.0~rc1 and Octez v21.0~rc2

It includes:

• New release page for Octez version 21
• Changelogs for Octez v21.0 rc1 and rc2
• Update of the openapi specifications

Why

Publishing day is comming

How

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