Include better information about documentation version

[matzipan]

Currently, while looking at valadoc for a specific library, it's unclear to which version of the library this documentation corresponds to. Sometimes methods get added, removed or changed between versions and it's hard to check if the valadoc manages your specific version.

Easy solution: display version information in the documentation.

Comprehensive solution: generate valadocs over multiple releases of the libraries, so you can navigate between them, similar to the way the python handles its core API documentation. This approach could also generate a diff of what has changed between two different versions.

[stsdc]

Hi @ricotz @tintou @colinkiama ! Are there any major blockers for this? At least simple label about version would be enough 🥺

[tintou]

@stsdc No major blocker, feel free to start working on this 😉

[stsdc]

Sure I can try 👍
But what you suggest to start from? Because that's a new thing for me 😅