CI: consider adding a smoke test of building a Docsy site from scratch

[chalin]

Consider adding a smoke test along the lines of what was proposed in https://github.com/google/docsy/issues/1075#issuecomment-1172432277.

Related:

• #726

[emckean]

I really need to come back to this! Hopefully soon ...

[chalin]

This also works:

hugo new site myproject && \
cd myproject && \
npm init -y && \
npm install --save-dev google/docsy autoprefixer postcss-cli
echo 'theme = "docsy"\nthemesDir = "node_modules"' >> hugo.toml
hugo