docs: final review polish for link health and source references

[johnxie]

Summary\n- fixed confirmed broken external links across tutorials and contributor templates\n- replaced stale project links with canonical current sources (Goose, Dify, Perplexica, Deer Flow, Fabric, AutoGen, RAGFlow, Botpress, OpenBB, PhotoPrism)\n- updated template placeholders to avoid shipping known-dead example links\n- aligned CONTRIBUTING contact links with available repo endpoints\n\n## Validation\n- broken_link_count=0

new_broken_link_count=0
resolved_link_count=0
missing_index_count=0
placeholder_summary_count=0
structure_counts={"docs_only": 0, "index_only": 0, "mixed": 0, "root_only": 185}\n- v2_index_count=110
All format_version: v2 indexes contain required section categories.\n- regenerated derived assets:\n - Wrote manifest: /Users/johnxie/Documents/GitHub/awesome-code-docs/tutorials/tutorial-manifest.json
tutorial_count=185
structure_counts={"docs_only": 0, "index_only": 0, "mixed": 0, "root_only": 185}\n - Wrote JSON: /Users/johnxie/Documents/GitHub/awesome-code-docs/discoverability/tutorial-index.json
Wrote llms.txt: /Users/johnxie/Documents/GitHub/awesome-code-docs/llms.txt
Wrote llms-full.txt: /Users/johnxie/Documents/GitHub/awesome-code-docs/llms-full.txt
Wrote directory markdown: /Users/johnxie/Documents/GitHub/awesome-code-docs/discoverability/tutorial-directory.md
tutorial_count=185\n - Wrote: /Users/johnxie/Documents/GitHub/awesome-code-docs/tutorials/README.md
tutorial_directories=185
tutorial_markdown_files=1678
tutorial_markdown_lines=477603
structure_counts={'root_only': 185, 'docs_only': 0, 'index_only': 0, 'mixed': 0}\n - Wrote: /Users/johnxie/Documents/GitHub/awesome-code-docs/TUTORIAL_STRUCTURE.md
Wrote: /Users/johnxie/Documents/GitHub/awesome-code-docs/CONTENT_GAPS_ANALYSIS.md\n- custom external-link sweep of markdown links (full repo + changed-file subset):\n - hard failures (HTTP 404/410): 0\n - remaining transient errors were non-deterministic network resets on otherwise-200 GitHub URLs