Local/CI documentation lacks navigation bar

[nomeata]

In #2449 we created a local, Motoko-only build of the docs, served at https://hydra.dfinity.systems/build/4427921/download/1/docs/language-guide/motoko.html

But that change broke the “real antora build” (fixed in #2487)

Now the local build has no navigation any more, which makes it mostly useless.

Will have to look again at some point. Probably need to locally modify the antora.yml before building, e.g. from the nix build script.