Add code of conduct markdown

[pnoltes]

Small PR that introduces a code of conduct document.

The document simply refers to the code of conduct of the ASF.
When reviewing, also read the referenced document to check if you agree with the code of conduct.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.52%. Comparing base (c6d0e2a) to head (c8dd7c4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #809      +/-   ##
==========================================
+ Coverage   91.44%   91.52%   +0.07%     
==========================================
  Files         234      234              
  Lines       28655    28655              
==========================================
+ Hits        26204    26226      +22     
+ Misses       2451     2429      -22     

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[PengZheng]

I have read the mentioned CoC word by word, and fully agree with it.

[rlenferink]

I noticed that the Celix repo on GitHub already points to the ASF wide code-of-conduct:

• https://github.com/apache/celix?tab=coc-ov-file#readme
• https://github.com/apache/.github/blob/main/.github/CODE_OF_CONDUCT.md

By making this copy we acknowledge that we have a copy and if the original is changed, we probably should update the copy in the Celix repo as well.

Having a copy can be considered (and IMO is) a good thing, because the whole organization .github folder etc. is GitHub specific, where any other repo copy (e.g. on Gitbox) otherwise won't point to the ASF code-of-conduct.

Should we add this file to the celix-site repo as well?