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

@@ -211,7 +211,7 @@ proof-state:
   needs: ["compile"]
   dependencies:
     - compile
-  impage: bbbrandenburg/alectryon-ci:latest
+  image: bbbrandenburg/alectryon-ci:latest
   script:
     - make alectryon
     - mv html ../with-proofs-and-proof-state