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 <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>