feat: GTFS diff schema v2

[cka-y]

This pull request introduces a new versioning structure for the GTFS Diff specification, documents the stable v1 (CSV) and draft v2 (JSON) formats, and adds comprehensive documentation and examples for both. The v2 specification is formalized with a detailed JSON Schema and a realistic example output, clarifying the structure and semantics of the new JSON-based diff format.

Specification Versioning and Documentation

• Added a "Specification versions" section to README.md, listing v1 (CSV, stable) and v2 (JSON, draft), and updated resource links to reflect the new formats.
• Added a complete specification document for v1 (CSV) format, including field definitions, ordering, examples, and rationale.

GTFS Diff v2 (JSON) Format

• Introduced a draft JSON Schema for GTFS Diff v2, defining the structure for metadata, summary, and per-file diffs, including row-level changes and truncation metadata. (spec/v2/json_schema/gtfs_diff_v2_schema.json)
• Added a realistic example output file for v2, demonstrating metadata, summary statistics, and detailed file diffs in the new JSON format. (spec/v2/examples/example_output.json)

[jcpitre]

Suggested change

	|id|String|Required|Uniquely identifies a row in the GFTS Diff file.|
	|id|String|Required|Uniquely identifies a row in the GTFS Diff file.|