fix: Serve manual under /lts and /latest URLs

[sabine]

Resolves #2344

• make documentation links point to /manual/lts

[Octachron]

I am not sure if we want to link to lts rather than latest: my issue is that the existence of a lts release distinct from the lastest release is temporary : it will last until OCaml 5 has restored full features and OS supports compared to OCaml 4. So hopefully, not beyond 2026 .

Another issue is that for the Atomic module, we definitively want to link a multicore enabled version, which means not the lts version which is a sequential dummy implementation of the interface.