Update Git proxy's governance to latest FINOS standard and formalize agreement reviews #1660

Git proxy's governance docs are:

• slightly behind the current FINOS standard (https://github.com/finos/software-project-blueprint/blob/main/CONTRIBUTING.md)
• exist within the website, rather than git repo: https://git-proxy.finos.org/docs/development/contributing
  • Repo version of teh doc is not necessarily in sync, but does refer to the website for governance details: https://github.com/finos/git-proxy/blob/main/CONTRIBUTING.md
• In need of an update to guide on what changes to Git Proxy should be review by a maintainer from a different firm to the one raising the PR (to ensure good governance, while limiting slow downs caused by reviews).

The original discussion ca be found here;; #1646 (comment)