Thanks for providing the smoke tests! They proved to be useful already!
Here comes some feedback:

1. Synopsis, retrieved via ./make-site.sh -h

   Creates a Docsy-themed site under SITE_NAME using the Hugo new command.
    Docsy is fetched as an NPM package from google/docsy in GitHub,
    unless the -l or -d HUGO_MOD flags are used.
   

   I don't see a -d flag, however. I guess this should be -s?
   This is now addressed in Smoke test: improvements #1800.

2. Small nit:

   ./make-site.sh -s foo
   ERROR: invalid argument to -s flag: FOO`
   

   This should be foo, not FOO. Can be fixed easily.
   This is now addressed in Smoke test: improvements #1800.

3. After the hugo build, there is this line on STDOUT:

   + [[ -n 1 ]]
   

   What does that mean?

4. After the npm install, there is this line on STDOUT:

   + [[ NPM == HUGO* ]]
   

   What is that meant for?

5. Running this command fails

   ./make-site.sh -fv v0.5.1 -s HUGO
   

   This was to be expected, since module depencendies were altered in v0.8.0 (Offer Docsy as a single Hugo Module #1120). Can we check the version given here and raise and error of below v0.8.0 with module installation? An alternative would be to mention this limitation in the synopsis.

6. Inside the smoke tests there is a matrix defined so that tests can be reused and executed on Linux and Windows machines and for NPM and hugo module build. This seems logical, it comes with one disadvantage, though: as soon as a smoke test for Linux fails, the corresponding smoke test on Windows is cancelled. For me this turned out to be unfortunate, so I decided to create individual action files for the different environments (see this commit in my docsy fork). Separate workflow also do have the advantage that individual smoke tests now can be easily enabled/disabled without touching the code.