[CI] Add smoke tests of building a Docsy site from scratch

[chalin]

Consider adding a smoke test along the lines of what was proposed in #1075 (comment):

$ hugo new site myproject
...
$ cd myproject
$ git init # required for the git submodule command to work
...
$ git submodule add --depth 1 https://github.com/google/docsy.git themes/docsy
...
$ echo 'theme = "docsy"' >> hugo.toml
$ (cd themes/docsy && npm install)
...
$ npm init -y
$ npm install -D autoprefixer postcss-cli postcss
...
$ hugo -v
Start building sites … 

Related:

• Add golden tests #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