docs: add pull request template & update contributing docs.

[qicz]

Fixes: #1315

[codecov]

Codecov Report

Merging #1314 (5a7598c) into main (6baaaf1) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1314      +/-   ##
==========================================
- Coverage   61.88%   61.85%   -0.03%     
==========================================
  Files          85       85              
  Lines       10852    10852              
==========================================
- Hits         6716     6713       -3     
- Misses       3692     3694       +2     
- Partials      444      445       +1     

see 2 files with indirect coverage changes

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[zirain]

EG use squash merge, do not push -f every time for better review.

[qicz]

EG use squash merge, do not push -f every time for better review.

I know squash will merge all commits into one commit. sometimes -f to fix codecov timeout or clarify the commit message.

[arkodg]

LGTM, this should make release note generation easier in the future