Add Doxygen comments to opentimelineio

[darbyjohnston]

As a follow up to #1753, this completes the first round of adding Doxygen comments. Some of the comments are a bit light on content, but hopefully provide a reasonable framework for adding more details in the future. Any feedback on improving the comments is welcome.

The changes in this PR are just comments, there are no functional changes to the API. There are some white space changes where the Doxygen comments changed the formatting.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.72%. Comparing base (c0e97b0) to head (93d471a).
Report is 52 commits behind head on main.

❌ Your changes status has failed because you have indirect coverage changes. Learn more about Unexpected Coverage Changes and reasons for indirect coverage changes.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1868      +/-   ##
==========================================
+ Coverage   84.11%   84.72%   +0.61%     
==========================================
  Files         198      177      -21     
  Lines       22241    12809    -9432     
  Branches     4687     1191    -3496     
==========================================
- Hits        18709    10853    -7856     
+ Misses       2610     1773     -837     
+ Partials      922      183     -739     

Flag	Coverage Δ
py-unittests	84.72% <ø> (+0.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/opentimelineio/anyDictionary.h	96.92% <ø> (+5.39%)	⬆️
src/opentimelineio/anyVector.h	91.42% <ø> (+7.55%)	⬆️
src/opentimelineio/composable.h	100.00% <ø> (ø)
src/opentimelineio/composition.h	83.87% <ø> (+18.35%)	⬆️
src/opentimelineio/effect.h	83.33% <ø> (-16.67%)	⬇️
src/opentimelineio/errorStatus.h	100.00% <ø> (+4.34%)	⬆️
src/opentimelineio/externalReference.h	100.00% <ø> (ø)
src/opentimelineio/generatorReference.h	40.00% <ø> (-60.00%)	⬇️
src/opentimelineio/imageSequenceReference.h	48.27% <ø> (-8.25%)	⬇️
src/opentimelineio/item.h	100.00% <ø> (ø)
... and 12 more

... and 114 files with indirect coverage changes

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[jhodges10]

This is awesome @darbyjohnston! Will try to find time to review today or tomorrow.