SCHEMA: Define JSON documents in schema

[effigies]

Follow up to #1233. This makes genetic_info.json and dataset_description.json objects in objects.documents instead of objects.metadata, which seems more appropriate.

One sticking point is how to indicate conditional requirement levels in an object, rather than a rule.

[codecov[bot]]

Codecov Report

Base: 88.65% // Head: 88.33% // Decreases project coverage by -0.31% :warning:

Coverage data is based on head (af09b93) compared to base (e70278c). Patch coverage: 20.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1353      +/-   ##
==========================================
- Coverage   88.65%   88.33%   -0.32%     
==========================================
  Files          11       11              
  Lines        1084     1089       +5     
==========================================
+ Hits          961      962       +1     
- Misses        123      127       +4     

Impacted Files	Coverage Δ
tools/schemacode/bidsschematools/render/tables.py	85.42% <20.00%> (-1.69%)	:arrow_down:

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