Update validity management code to new format

[ggmarshall]

legend-exp/legend-data-format-specs#10

[gipert]

.yaml is more common than .yml, I think. I would propose to stick to that.

[gipert]

can you please update the docs?

[gipert]

...including adding the essential docstrings to catalog.py?

[codecov]

Codecov Report

Attention: Patch coverage is 73.17073% with 11 lines in your changes missing coverage. Please review.

Project coverage is 72.02%. Comparing base (838ca85) to head (b460921).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
src/legendmeta/catalog.py	78.57%	6 Missing ⚠️
src/legendmeta/police.py	16.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
+ Coverage   71.95%   72.02%   +0.06%     
==========================================
  Files           8        8              
  Lines         813      840      +27     
==========================================
+ Hits          585      605      +20     
- Misses        228      235       +7     

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