Docs, v20: Documentation for Octez v20.0~rc1

What

Documentation for Octez v20.0~rc1.

To be merged only on release day !

Why

How

• Introduce new release page
• Add RC openapi specifications
• Snapshot changelogs

Manually testing the MR

Check the generated doc

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