ci(doc): cache the opam switch to skip recompiling the dependency tree

[balat]

Problem

The Documentation workflow takes ~5 min per push, of which ~4.5 min is Install dependencies — recompiling the dependency tree plus wodoc and odoc from source every time. setup-ocaml@v3 only caches the base switch (the compiler).

Step	Duration
Set up OCaml	~20 s
Install dependencies	4 min 23 s
Build documentation (dev)	~2 s

Fix

Cache the whole _opam switch with actions/cache, keyed on the *.opam files (the dependency set) and wodoc's resolved HEAD commit. On a cache hit the install step is skipped; ocsigenserver itself is still rebuilt from the checkout by dune build @doc in the next step, so the docs stay current. The cache rebuilds once when an *.opam file or wodoc's HEAD move.

This mirrors the pilot on ocsigen.github.io (4 min → ~1 min). First run on this branch is a cache miss (populates the cache); the speed-up shows on the next run.

Doc-CI pilot for the API-project pattern. Once confirmed it rolls out to toolkit, start, tyxml, i18n, ocsipersist, lwt, reactiveData (eliom too, once its unrelated wasm_of_ocaml build breakage is fixed).