Add/update DCO, CoC, contributing ... files

[mmlb]

Description

Update non code files to be uniformish amongst all repos.

Why is this needed

Ensures our repos are all licensed correctly and contain similar/minimum
contribution info files.

[codecov]

Codecov Report

Merging #515 (480ade7) into master (04b1e9e) will not change coverage.
The diff coverage is n/a.

❗ Current head 480ade7 differs from pull request most recent head 009342d. Consider uploading reports for the commit 009342d to get more accurate results

@@           Coverage Diff           @@
##           master     #515   +/-   ##
=======================================
  Coverage   33.61%   33.61%           
=======================================
  Files          44       44           
  Lines        3385     3385           
=======================================
  Hits         1138     1138           
  Misses       2150     2150           
  Partials       97       97           

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