docs: contrib/issue templates (#2377)

* docs: move helpers to .github where allowed

* docs: more guidelines in CONTRIBUTING

* chore: update issue templates

* fix: review from @bstaletic

* refactor: a few points from @rwgk

* docs: more touchup, review changes

.github/ISSUE_TEMPLATE/bug-report.md

@@ -0,0 +1,28 @@
+---
+name: Bug Report
+about: File an issue about a bug
+title: "[BUG] "
+---
+
+
+Make sure you've completed the following steps before submitting your issue -- thank you!
+
+1. Make sure you've read the [documentation][]. Your issue may be addressed there.
+2. Search the [issue tracker][] to verify that this hasn't already been reported. +1 or comment there if it has.
+3. Consider asking first in the [Gitter chat room][].
+4. Include a self-contained and minimal piece of code that reproduces the problem. If that's not possible, try to make the description as clear as possible.
+    a. If possible, make a PR with a new, failing test to give us a starting point to work on!
+
+[documentation]: https://pybind11.readthedocs.io
+[issue tracker]: https://github.com/pybind/pybind11/issues
+[Gitter chat room]: https://gitter.im/pybind/Lobby
+
+*After reading, remove this checklist and the template text in parentheses below.*
+
+## Issue description
+
+(Provide a short description, state the expected behavior and what actually happens.)
+
+## Reproducible example code
+
+(The code should be minimal, have no external dependencies, isolate the function(s) that cause breakage. Submit matched and complete C++ and Python snippets that can be easily compiled and run to diagnose the issue.)