[ocaml] Don't rebuild documentation for each tag every time.

[franziskuskiefer]

With the recent change in https://github.com/cryspen/hacl-packages/commit/81303b83a54a92d3b5f54f1b8ddbea60438cc2bf the built of old ocaml tags doesn't work anymore. This has always been inefficient. Instead the old documentation should be reused.