Add doc summarizing info on v2 schemas

[lmolkova]

Cleanup and document v2 schemas:

• add head_schema_url and baseling_schema_url to diff
• add missing JSON schemas
• rename forge to materialized in json-schema command
• add md overview

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.6%. Comparing base (bfea2e5) to head (60fd86b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1106     +/-   ##
=======================================
- Coverage   81.6%   81.6%   -0.1%     
=======================================
  Files        113     113             
  Lines       9676    9682      +6     
=======================================
+ Hits        7901    7904      +3     
- Misses      1775    1778      +3     

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[lquerel]

LGTM