deps: move zlib maintenance info to guides

deps/zlib/README.md is not part of the upstream zlib, it is a Node.js
specific addition describing how to maintain zlib and should be in
doc/guides/.

PR-URL: #31800
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Beth Griggs <[email protected]>

Author: sam-github