Change URL structure to support multiple languages and app-version-specific documentation

[cketti]

At some point we'll hopefully support documentation in multiple languages and maybe documentation for specific app versions, e.g. allow users to read/link to the documentation for K-9 Mail X even though K-9 Mail X+1 is the current version. This is not a priority right now, but we should prepare the URLs to support this later.

I propose that instead of a URL like

https://docs.k9mail.app/setup/installation/

we use something like

https://docs.k9mail.app/en/current/setup/installation/