docs: refactor and expand documentation structure (#3617)- Remove top-level README.md and move content into docs/ - Rewrite and expand docs/README.md with clearer structure and links - Add docs/OWNERS for area/docs label - Add detailed guides: core-concepts.md, designs/README.md,guides/README.md, guides/FAQ.md- Expand contributing.md with workflow steps - Add jobs information section - move deprecated `release-checker` to `tools` folder---------Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com> (commit: 65d3ed6)