Sphinx docs gh-pages implementation

[tjex]

As discussed, this PR implements the new docs solution into the main zk repo.

The main points for this:

• docs info is kept in sync with zk source code by design (and will be packaged up with releases).
• having docs in the main repo means less friction to update docs in tandem with fixes / feature PRs. I.e, more chance the docs remain up to date.