docs: add plugin discovery docs

[johnnygreco]

📋 Summary

Adds a dedicated Discover Plugins docs page for the new plugin CLI workflow. The page explains catalog-based discovery, package aliases, installation and uninstall commands, alternate catalogs, and the path for future community plugins.

🔗 Related Issue

N/A — stacked on #618.

🔄 Changes

• Add Discover Plugins docs for both MkDocs and Fern.
• Rename the docs route from plugins/available to plugins/discover.
• Update plugin overview pages and nav entries to point readers at the catalog workflow.
• Remove the old Fern available redirect because this route has not shipped yet.

🧪 Testing

• git diff --check passes
• fern check passes
• uv run mkdocs build passes
• Unit tests N/A — docs-only change
• E2E tests N/A — docs-only change

✅ Checklist

• Follows commit message conventions
• Commits are signed off (DCO)
• Architecture docs updated (N/A — user-facing docs only)

[greptile-apps]

Greptile Summary

This PR replaces the placeholder "Available Plugins" docs page with a new "Discover Plugins" page covering the catalog-based CLI workflow, and updates nav entries and cross-links across MkDocs and Fern accordingly.

• Deletes the available.md/available.mdx placeholder ("Coming Soon") and introduces discover.md/discover.mdx with full CLI command documentation, catalog mechanics, and community plugin guidance.
• Updates navigation in mkdocs.yml, fern/versions/latest.yml, v0.5.8.yml, and v0.5.9.yml, and removes the now-unnecessary Fern redirect for the unshipped /plugins/available route.
• Refreshes overview.md/overview.mdx to point readers at the new Discover page instead of the old Available list.

Confidence Score: 5/5

Safe to merge — purely additive documentation with no code changes.

All changes are documentation only: a new page, nav updates, cross-link corrections, and removal of a never-shipped placeholder and its stale redirect. Content is internally consistent between the MkDocs and Fern versions, and the PR description accounts for the stacking dependency on #618.

No files require special attention.

Important Files Changed

Filename	Overview
docs/plugins/discover.md	New MkDocs page with full catalog-based plugin discovery docs; content is accurate and well-structured
fern/versions/v0.5.8/pages/plugins/discover.mdx	New Fern page mirroring discover.md; uses correct Fern component syntax (Note, absolute paths) and is consistent with the MkDocs version
fern/versions/v0.5.8.yml	Swaps the nav entry from available.mdx (placeholder) to discover.mdx; consistent with latest.yml and v0.5.9.yml
fern/docs.yml	Removes the /plugins/available redirect that was never needed for a shipped route
mkdocs.yml	Single nav entry rename from available.md to discover.md; correct
docs/plugins/overview.md	Minor wording tweak and cross-link updated from available.md to discover.md
fern/versions/v0.5.8/pages/plugins/overview.mdx	Removes the pip install code block from the usage section and redirects readers to the new Discover page

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[plugins/overview] -->|"See Discover Plugins"| B[plugins/discover]
    B --> C["NVIDIA catalog\n(default)"]
    B --> D["Custom catalogs\n(--catalog flag)"]
    C --> E["data-designer plugin list/search/info"]
    D --> E
    E --> F["data-designer plugin install"]
    F --> G["Entry point discovery\ndata_designer.plugins"]
    G --> H["Runtime plugins available"]
    B --> I["Community plugins\n(PyPI + GitHub issue)"]

    style B fill:#4CAF50,color:#fff
    style A fill:#2196F3,color:#fff

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[github-actions]

MkDocs preview: https://6229b46c.dd-docs-preview.pages.dev

Fern preview: https://nvidia-preview-pr-642.docs.buildwithfern.com/nemo/datadesigner​

Notebook tutorials are rendered without execution outputs in previews.