Adopt OCaml's code of conduct

[balat]

This PR proposes to add the OCaml Code of Conduct. Full text is available at https://github.com/ocaml/code-of-conduct, and a discussion around it is at https://discuss.ocaml.org/t/ocaml-community-code-of-conduct/10494.

[art-w]

Thanks!

[samoht]

Would be nice to point to a list of contacts in to reach out in case of issue. I suggest pointing to https://github.com/orgs/mirage/teams/irmin-dev

[balat]

It seems that this list is private. What about putting all names and email addresses of current Irmin team members? Or do you prefer a more restristed list?

[samoht]

Having names and email addresses is indeed better - I'm happy to have mine.

[balat]

Just updated the PR

[codecov-commenter]

Codecov Report

Merging #2255 (1c30a45) into main (0075198) will increase coverage by 0.04%.
The diff coverage is 92.68%.

❗ Current head 1c30a45 differs from pull request most recent head 3d25bdd. Consider uploading reports for the commit 3d25bdd to get more accurate results

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@@            Coverage Diff             @@
##             main    #2255      +/-   ##
==========================================
+ Coverage   68.19%   68.23%   +0.04%     
==========================================
  Files         137      137              
  Lines       16674    16695      +21     
==========================================
+ Hits        11371    11392      +21     
  Misses       5303     5303              

Impacted Files	Coverage Δ
bench/irmin-pack/trace_stat_summary_utils.ml	72.30% <0.00%> (ø)
src/irmin-pack/unix/store.ml	65.66% <80.00%> (+0.10%)	⬆️
src/irmin/tree.ml	81.49% <95.45%> (+0.32%)	⬆️
bench/irmin-pack/bench_common.ml	23.52% <100.00%> (ø)
bench/irmin-pack/trace_collection.ml	83.33% <100.00%> (+0.21%)	⬆️
src/irmin-pack/unix/file_manager.ml	85.78% <100.00%> (ø)
src/irmin-pack/unix/gc.ml	72.89% <100.00%> (+0.25%)	⬆️
src/irmin-test/store_watch.ml	30.76% <100.00%> (ø)
src/irmin/import.ml	75.92% <100.00%> (+0.92%)	⬆️
src/irmin/logging.ml	95.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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