[CI] Smoke test Docsy-based site using Hugo-modules

[chalin]

Contributes to:

• [CI] Improvements in CI and infra 24Q1 #1780

It may be necessary to only run these smoke tests over PRs/branches in this repo. I've tried fetching from a branch in my fork, for example, but Go complains (rightly) that modules paths aren't as expected:

$ hugo mod get github.com/chalin/docsy@chalin-im-blog-post-lastmod-2024-01-15
go: downloading github.com/chalin/docsy v0.0.0-20240115234354-346c42b292cc
go: github.com/chalin/docsy@chalin-im-blog-post-lastmod-2024-01-15 (v0.0.0-20240115234354-346c42b292cc) requires github.com/chalin/docsy@v0.0.0-20240115234354-346c42b292cc: parsing go.mod:
        module declares its path as: github.com/google/docsy
                but was required as: github.com/chalin/docsy

And I can't seem to specify a replace rule from the command line.

[chalin]

I've found a way to work from PR branches.