CI: run docs, publish docs

[timotheecour]

TODO in future work

• make sure some index.html exists so we can point directly to https://nim-lang.github.io/fusion instead of https://nim-lang.github.io/fusion/theindex.html
• link to https://nim-lang.github.io/fusion from nim repo

[alaviss]

You'll have to do the cd again, the environment is reset between steps.

[timotheecour]

You'll have to do the cd again, the environment is reset between steps.

done