Use alectryon to generate doc with proof state

Let's use this awesome tool: https://github.com/cpitclaudel/alectryon

While at it, also update the CI configuration to minimize the jobs that run in wip- branches.

.gitlab-ci.yml

@@ -213,6 +213,7 @@ proof-state:
     - compile
   image: bbbrandenburg/alectryon-ci:latest
   script:
+    - eval $(opam env "--switch=${COMPILER_EDGE}" --set-switch)
     - make alectryon
     - mv html ../with-proofs-and-proof-state
   artifacts: