My assessment is that this is unlikely to build in the short term.
The service that builds the documentation needs to be developed further to enable more different build environments. This concerns other Linux environments as well as all the other Tier-1 OCaml targets (macOS, Windows, FreeBSD).
Thank you for bringing this up because it helps us highlight how this work is important. In the meantime, I opened an issue here: Building Docs for Packages When Depext is Unavailable · Issue #146 · ocurrent/ocaml-docs-ci · GitHub