Add CODEOWNERS and update roles

[sethrj]

This adds a preliminary set of "code owners" (i.e. experts/responsible reviewers) and a brief discussion of its meaning. It also updates the roles to reflect our current practices, and removing "maintainers" (which didn't work as expected: being able to approve a PR required the same "write" privilege as being able to hit the "merge" button). It also proposes the "affiliate" role to replace "student".

@celeritas-project/core Please add code suggestions to add or remove yourself, adding new directories as needed.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 84.86%. Comparing base (068a4ea) to head (87ce8ab). :warning: Report is 12 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2113      +/-   ##
===========================================
- Coverage    84.87%   84.86%   -0.01%     
===========================================
  Files         1255     1255              
  Lines        44185    44179       -6     
  Branches     16501    16499       -2     
===========================================
- Hits         37503    37494       -9     
  Misses        4725     4725              
- Partials      1957     1960       +3     

see 8 files with indirect coverage changes

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[github-actions[bot]]

Test summary

 5 359 files   8 624 suites   17m 15s :stopwatch:  2 082 tests  2 056 :white_check_mark:  26 :zzz: 0 :x: 30 213 runs  30 076 :white_check_mark: 137 :zzz: 0 :x:

Results for commit 87ce8ab7.

:recycle: This comment has been updated with latest results.