Set up a `zulip-archive` repo for search engine indexing

[jaimergp]

https://github.com/zulip/zulip-archive exports public Zulip messages via a GH Pages site. Found about this recommendation in jupyter/governance#182 (comment).

Looks simple to set up via GHA, we can just put it in a repo and forget about it.

[jaimergp]

Example site: https://leanprover-community.github.io/archive/stream/113488-general/topic/Notation.20for.20the.20Lean.20InfoView.html