feat: Adds versioned documentation

[jknndy]

Resolves #1663

Not related changes

• Cache pip on run

publish.yaml

• Adds permissions for write to allow doc updates
• Install doc dependencies
• Add mike to automatically deploy versioned docs
• Release flow update to allow versioned docs

Docs

• Added a “Publishing Versioned Docs” section to the contributor guide, explaining that releases automatically publish docs with Mike and how to run the same commands locally when needed
• Highlighted in the supported-sites page that older site lists remain accessible via versioned URLs (e.g., /15.8.0/getting-started/supported-sites/) for compatibility checks

@tomtjes , not sure if you're familiar with mike but if you want to take a look any feedback would be appreciated!

[tomtjes]

Awesome! Don't know anything about mike though, sorry

[hhursev]

Very important PR! This looks very elegant. I'll take a look right now