Update Config classes / testing / documentation

[jsiirola]

Fixes # .

Summary/Motivation:

This PR was motivated by two new features in the Config system:

• ConfigDict.add() should accept keyword arguments so that users can specify modifiers like visibility without explicitly creating the ConfigValue
• ConfigDict should derive from MutableMapping so that methods like update() are defined / available.

As part of implementing that, this PR made a number of other improvements

• [Performance] Split ConfigBase.__call__ so that the ConfigDict-specific logic is in an overload method on ConfigDict
• [Performance] Avoid unnecessary calls to generate_documentation()
• [Performance] re-use method documenter in document_class_CONFIG
• [bugfix] fix logic when trapping recursion errors in domain pickler
• [feature] Expose additional classes from enum (Flag, IntFlag, EnumType)
• Update / expand unit testing
• Significant update / expansion on documentation

Changes proposed in this PR:

• (see above)

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.43%. Comparing base (a079b25) to head (a7d7b5a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3790      +/-   ##
==========================================
+ Coverage   89.41%   89.43%   +0.01%     
==========================================
  Files         909      909              
  Lines      105579   105586       +7     
==========================================
+ Hits        94407    94426      +19     
+ Misses      11172    11160      -12     

Flag	Coverage Δ
builders	29.10% <72.88%> (-0.02%)	⬇️
default	86.05% <89.83%> (?)
expensive	35.75% <76.27%> (?)
linux	86.74% <100.00%> (-2.45%)	⬇️
linux_other	86.74% <100.00%> (+0.01%)	⬆️
osx	82.89% <89.83%> (+<0.01%)	⬆️
win	84.99% <89.83%> (+<0.01%)	⬆️
win_other	84.99% <89.83%> (+<0.01%)	⬆️

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[mrmundt]

Ooo I like this.