Add API versioning

[sabineschaller]

For the OpenAPI schemas, we need to add versioning just like we had it on readme.

We want to add a dropdown to select the API version. It can be parsed from the yaml file. Additionally, we need to always show what's on main as the "version in development" (somebody find a better term for that).

[huijing]

Threads to follow:

• Support for Versioned Docs withastro/starlight#957
• https://discord.com/channels/830184174198718474/1117052663520034866

[sabineschaller]

We decided to only have 2 versions. A stable one and a latest one which we can make changes to.

[melissahenderson]

Considering the age of this issue, we will need to discuss whether this is still something we want to do.

[melissahenderson]

@sabineschaller Is this something we should proceed with? Should the user docs only display the stable version, or do we want to make both versions available?