update issue/pr templates and link to mesa contributor guide

[wang-boyu]

This PR updates issue/pr templates to be consistent with Mesa. Also adds a link to mesa contributor guide for broader contribution process and more information.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.09%. Comparing base (47cf55f) to head (1d77f2d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #315   +/-   ##
=======================================
  Coverage   78.09%   78.09%           
=======================================
  Files          10       10           
  Lines         995      995           
  Branches      158      158           
=======================================
  Hits          777      777           
  Misses        177      177           
  Partials       41       41           

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