Add Zsh docs

[QuarticCat]

Feature request

Summary

We already have Bash docs, then why not Zsh?

https://zsh.sourceforge.io/Doc/

[mrlee14]

exactly! +1

[intelfx]

Having indexed Zsh docs would be very useful, esp. given that they are quite voluminous (much bigger than Bash) and looking something in the (un-indexed) official HTML docs can be quite a pain.