docs(extraction): add Customize & extend hub page

[kheiss-uwzoo]

Summary

• Add published Customize & extend page that surfaces UDF, custom graph pipeline, custom VDB, and coming-soon custom embedding paths with xrefs to repository guides.
• Replace section 8 nav link (previously graph README only) with the new page; redirect legacy \user-defined-functions\ and \user-defined-stages\ slugs.
• Cross-link from \concepts.md,
  eleasenotes.md, and \�dbs.md.

Follow-up to Randy's doc IA feedback (separate from #2276 starter-kits work).

Test plan

• \mkdocs build -f docs/mkdocs.yml --strict\
• \check-nrl-doc-leakage.ps1\
• Sidebar section 8 opens \customize-extend.md\
• Legacy /extraction/user-defined-functions/\ redirects after republish

Related

• docs(extraction): move Starter kits to examples/README.md (#2266 follow-up) #2276 — starter kits → \examples/README.md\ (separate PR)

[greptile-apps]

Greptile Summary

This PR adds a new Customize & extend hub page (customize-extend.md) that consolidates all extension paths (task configuration, UDFs, custom graph pipelines, custom VDB adapters) into one doc-site page, replacing the previous Section 8 nav entry which pointed directly to a GitHub README. Legacy slug redirects (user-defined-functions, user-defined-stages, customize-and-extend) are wired in mkdocs.yml, and concepts.md / vdbs.md are updated to cross-link the new page.

• New page (customize-extend.md): structured hub with four extension-path sections, anchor IDs, and a decision table; all internal links use relative paths that resolve within the doc site.
• mkdocs.yml: Section 8 nav entry converted from external GitHub link to local page; four new redirect_maps entries added, two of which include anchor fragments (handled correctly by the mkdocs-redirects HTML meta-refresh).
• concepts.md / vdbs.md: two GitHub deep-link references replaced with relative doc-site cross-references, and a new Related Topics entry added in vdbs.md.

Confidence Score: 5/5

Documentation-only change; no code paths, APIs, or data flows are altered. Safe to merge.

All changes are additive doc content and nav/redirect config. The mkdocs build --strict test in the checklist covers broken links and malformed YAML. The two open items (missing releasenotes cross-link, absent coming-soon embeddings section) are minor editorial gaps that do not block correct rendering of any existing or new page.

No files require special attention; the minor editorial gaps in customize-extend.md are low risk and do not affect the build.

Important Files Changed

Filename	Overview
docs/docs/extraction/customize-extend.md	New hub page for extension paths (UDFs, graph pipelines, custom VDB, task config); well-structured with anchor IDs and cross-references. PR description mentions a coming-soon custom embedding section that is absent.
docs/mkdocs.yml	Section 8 nav entry replaced from GitHub external link to new local page; four new redirects added including two with anchor fragments, which the redirects plugin handles correctly at runtime.
docs/docs/extraction/concepts.md	Two inline GitHub deep-links replaced with relative doc-site cross-references to customize-extend.md; straightforward and correct.
docs/docs/extraction/vdbs.md	Cross-link to customize-extend.md added inline and in Related Topics; small additive change, no issues.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Section 8 nav entry\n'8. Customize & extend'"] --> B["customize-extend.md\n(new hub page)"]

    B --> C["Start with task configuration\n#start-with-task-configuration"]
    B --> D["User-defined functions (UDFs)\n#user-defined-functions-udfs"]
    B --> E["Custom graph pipelines\n#custom-graph-pipelines"]
    B --> F["Custom vector databases\n#custom-vector-databases"]

    G["concepts.md"] -->|cross-link| B
    H["vdbs.md"] -->|cross-link| B

    I["Legacy: user-defined-stages.md"] -->|redirect| B
    J["Legacy: user-defined-functions.md"] -->|redirect| D
    K["Legacy: user-defined-functions/index.md"] -->|redirect| D
    L["Legacy: customize-and-extend.md"] -->|redirect| B

    D -->|xref| M["GitHub: graph README #using-udfoperator"]
    D -->|xref| N["GitHub: examples/udfs"]
    E -->|xref| O["GitHub: graph README #nemo-retriever-graph"]
    F -->|xref| P["vdbs.md"]
    F -->|xref| Q["GitHub: building_vdb_operator.ipynb"]

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
flowchart TD
    A["Section 8 nav entry\n'8. Customize & extend'"] --> B["customize-extend.md\n(new hub page)"]

    B --> C["Start with task configuration\n#start-with-task-configuration"]
    B --> D["User-defined functions (UDFs)\n#user-defined-functions-udfs"]
    B --> E["Custom graph pipelines\n#custom-graph-pipelines"]
    B --> F["Custom vector databases\n#custom-vector-databases"]

    G["concepts.md"] -->|cross-link| B
    H["vdbs.md"] -->|cross-link| B

    I["Legacy: user-defined-stages.md"] -->|redirect| B
    J["Legacy: user-defined-functions.md"] -->|redirect| D
    K["Legacy: user-defined-functions/index.md"] -->|redirect| D
    L["Legacy: customize-and-extend.md"] -->|redirect| B

    D -->|xref| M["GitHub: graph README #using-udfoperator"]
    D -->|xref| N["GitHub: examples/udfs"]
    E -->|xref| O["GitHub: graph README #nemo-retriever-graph"]
    F -->|xref| P["vdbs.md"]
    F -->|xref| Q["GitHub: building_vdb_operator.ipynb"]

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