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.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.31%. Comparing base (75592b7) to head (12df000).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #124   +/-   ##
=======================================
  Coverage   88.31%   88.31%           
=======================================
  Files          19       19           
  Lines        1429     1429           
=======================================
  Hits         1262     1262           
  Misses        167      167           

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