Governance process been actualized. (#82736)
Changes:
* form for topics proposals for Core maintainers review been added
* merge_rules.json file specified as spruce of truth for the list of maintainers (since it is the file that actually defines permissions)
* responsibilities of the module maintainers are added (as per the last core maintainers meeting)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82736
Approved by: https://github.com/svekars, https://github.com/soumith
