docs: add validation tools guide

[0xWeakSheep]

Title:
docs: add validation tools guide

Body:

Summary

• Add a new Validation Tools guide covering A2A Inspector, TCK, and ITK.
• Explain when to use each tool and how they fit into agent, protocol, SDK, and CI validation workflows.
• Add the new guide to the Resources navigation and update the roadmap validation section to include ITK.

Why

Users need a clearer entry point for understanding what TCK and ITK are, when to use them, and how they relate to Inspector-based debugging and SDK integration testing.

Fixes #1883

Validation

• git diff --check
• npx --yes markdownlint-cli@latest docs/validation-tools.md docs/roadmap.md --config .github/linters/.markdownlint.json
• /tmp/a2a-docs-venv/bin/mkdocs build --site-dir /tmp/a2a-site

Note: /tmp/a2a-docs-venv/bin/mkdocs build --strict --site-dir /tmp/a2a-site still reports existing repository warnings for generated SDK/spec artifacts and existing specification anchors. The new validation tools page does not introduce strict-mode warnings.

[gemini-code-assist]

Code Review

This pull request introduces a new documentation page, docs/validation-tools.md, which provides a comprehensive overview of the A2A validation tools (A2A Inspector, TCK, and ITK). It also updates docs/roadmap.md to reference this new guide and registers the page in mkdocs.yml. The feedback suggests a minor formatting improvement in the new documentation file to align the indentation of continuation lines in a list to two spaces for consistent markdown rendering.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[gemini-code-assist]

The continuation lines for these list items are indented with 4 spaces instead of 2 spaces. Aligning them to 2 spaces (matching the list marker's text start) ensures consistent markdown rendering and adheres to standard markdown style guidelines.

Suggested change

	- Agent implementation repositories can run the TCK against a local test server
	and publish the generated `reports/` artifacts.
	- SDK repositories can run ITK scenarios against a "current" SDK checkout and
	stable reference SDK agents.
	- Nightly pipelines can run broader ITK matrices and publish metrics for the
	centralized dashboard.
	- Agent implementation repositories can run the TCK against a local test server
	and publish the generated reports/ artifacts.
	- SDK repositories can run ITK scenarios against a "current" SDK checkout and
	stable reference SDK agents.
	- Nightly pipelines can run broader ITK matrices and publish metrics for the
	centralized dashboard.