docs: fix JSON schema download link

[coyaSONG]

Description

docs/definitions.md links to a version-specific spec/a2a.json artifact that
is intentionally generated during documentation builds and therefore is not
available from the GitHub source view. Keep that relative link as the primary
download so versioned documentation continues to serve its matching schema,
and add an explicitly labeled link to the official published latest artifact
for readers viewing the Markdown source on GitHub.

Validation

• npx --yes markdownlint-cli docs/definitions.md --config .github/linters/.markdownlint.json
• .doc-venv/bin/mkdocs build
• Confirmed the rendered site/definitions/index.html contains both the
  version-specific relative URL and the official latest fallback URL
• Confirmed https://a2a-protocol.org/latest/spec/a2a.json returns HTTP 200
• git diff --check

Checklist

• Follow the CONTRIBUTING Guide.
• Make your Pull Request title follow the Conventional Commits specification and repository rules (e.g., docs(spec): for docs/specification.md, docs: for other docs, and feat: / fix: reserved for a2a.proto). See CONTRIBUTING.md for details.
• Ensure the tests and linter pass (Run bash scripts/format.sh from the repository root to format)
• Appropriate docs were updated (if necessary)

AI assistance

OpenAI Codex was used to inspect the existing schema-generation pipeline,
prepare this documentation change, address review feedback about versioned
documentation, and run the validation listed above.

Fixes #2057 🦕

[gemini-code-assist]

Code Review

This pull request updates the download link for the a2a.json schema in docs/definitions.md from a relative path to an absolute URL. The reviewer points out that using an absolute URL pointing to the latest version breaks versioning for older documentation versions, and suggests reverting to the relative path to ensure MkDocs can resolve the version-specific schema correctly.

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[msampathkumar]

JLYK. Today this url refers to https://a2a-protocol.org/latest/spec/a2a.proto