Add SUPPORT document, update other GitHub templates (#23778)

Summary:
See https://help.github.com/en/articles/adding-support-resources-to-your-project. The SUPPORT file is surfaced by GitHub in several useful places. The goal is to increase discoverability of all support resources, giving the reader an opportunity to find the help they need before they open an issue in the              repository. A secondary goal is to use a friendlier tone when communicating our processes.

The issue templates have been standardized to follow the format used by GitHub's template tool. A bug in the "Regressions" template, due to its use of multiple labels, is fixed as well.
The bot will explain how a changelog can be fixed when it nags a PR.

[General][Added] - Added SUPPORT resources, friendlier messaging.
Pull Request resolved: https://github.com/facebook/react-native/pull/23778

Differential Revision: D14333911

Pulled By: hramos

fbshipit-source-id: d4b3e75194ea7d5c932dd8df2393d5b189f30f35