Add API versioning

[Sabine Schaller]

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).

[Chen Hui Jing]

Threads to follow:

• withastro/starlight#957
• https://discord.com/channels/830184174198718474/1117052663520034866

[Sabine Schaller]

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