🤖 Extend the documentation workflow

[pschmitt]

We need CI for the really boring stuff: zsdoc, manpage, PDF. Otherwise they will get out of sync. Should be fairly trivial to add.
In CI we can generate all of these and ship them to the repo.

It think these artifacts can be stored in the gh-pages branch. Keeping them as build artifacts is sadly not an option because of the max lifetime of these (90 days?). Let me know if there's a more appropriate place for these.

TODO:

• Rename and extend the gh-pages workflow
• Move the files living in the documentation branch (mkdocs) to the main one
• On-push generation of the zsdoc
• On-push generation of the PDF
• Remove the *.adoc, zsdoc/ etc from the main branch
• Update any link pointing to the aforementioned doc artifacts
• Update instructions in doc/README.md

Refs:

• 📚 Update manpage, zsd, README TOC and add instructions for docs #89
• CI #42
• 🤖 Check if zsd docs are up to date #117

[daehyeok]

#127 Automatically parsing and update package list also will be good.
Do you have any plan to use other language for automation(e.g Python)?