expected_assets checks should validate HTML docs were generated #4506

See nodejs/node#66571.

Presently our expected_assets checks during release promotion are only checking a few things under docs, e.g. for v26.x:

So we did not catch that no HTML files were generated.

I don't know the historical reason for only checking the top level docs (speculating here, but maybe time and/or the annoyance of listing out all 200+ files in each expected asset list?), but it does appear to be deliberate, e.g.