v2: polish resolved and materialized schemas and document them

[lmolkova]

Cleanup and document v2 schemas:

• add file_format: replaces version: "2" in definition schema, matches "2/resolved", "2/materialized" in resolved and materialized respectively.
  • version could be confusing (version of semconv or version of file?)
• add schema_url to resolved and materialized schemas
• remove registry_url from v2 Registry (it's top-level prop, also applies to refinements)
• fix json-schema command for definition and resoved ones
• add JSON schemas and md overview

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.1%. Comparing base (4d4caa5) to head (d73fe7a).

Files with missing lines	Patch %	Lines
crates/weaver_forge/src/v2/registry.rs	50.0%	2 Missing ⚠️
crates/weaver_semconv/src/semconv.rs	66.6%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1106     +/-   ##
=======================================
- Coverage   79.1%   79.1%   -0.1%     
=======================================
  Files        102     102             
  Lines       7952    7958      +6     
=======================================
+ Hits        6294    6298      +4     
- Misses      1658    1660      +2     

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