Document the plugin tap contract, workflows, and ecosystem boundaries

Parent epic: #15

Depends on: #16, #18, #19, #20, #21, #22, #24

Why

The repo currently documents plugin authoring, workflow, and release basics, but not the full tap contract or how DDPlugins functions as both the default NVIDIA tap and a reference tap repo. This issue turns the contracts from the other issues into maintainer-facing and consumer-facing docs.

Documentation changes

Add or update these docs:

1. docs/taps.md

   • Defines tap discovery vs runtime entry-point discovery.
   • Includes the schema v2 JSON example from #16.
   • Lists required schema fields and source object variants.
   • States the default NVIDIA tap URL exactly:
     https://raw.githubusercontent.com/NVIDIA-NeMo/DataDesignerPlugins/main/catalog/plugins.json
   • Explains that external taps may use any unauthenticated raw JSON URL or local path matching schema v2.
   • Explains mutability of the raw main URL and tag/SHA pinned alternatives.

2. README.md

   • Adds a short “Plugin Tap” section.
   • Links to docs/taps.md.
   • Shows the default NVIDIA tap URL.

3. docs/authoring.md

   • Explains that package metadata, plugin docs, entry points, compatibility dependencies, and tap config feed generated catalog entries.
   • Explains ddp new --type column-generator|seed-reader|processor once #22 lands.
   • Explains external tap authoring at a high level: configure [tool.ddp.tap], scaffold plugin, run make all, publish raw JSON catalog.

4. docs/workflow.md

   • States that catalog/plugins.json is the machine-readable tap artifact.
   • States that maintainers regenerate it with make catalog and validate with make check-catalog / make check.
   • States that human docs link to the raw JSON catalog but do not deliver the machine catalog.

5. docs/releasing.md

   • Documents per-plugin tags: {package}/v{version}.
   • Explains that PyPI source installs use source.package and package.version.
   • Explains that Git source installs use repository-git-url, release ref, and package.path as subdirectory.
   • Explains that multi-plugin packages release all of their entry points together.
   • Documents the strengthened release validation from #20.

6. zensical.toml

   • Adds the new tap docs page to navigation outside the generated plugin docs block.

Acceptance criteria

• A consumer can find the default NVIDIA tap URL and understand every schema v2 field.
• Docs explain that the tap artifact is raw JSON, not an HTML page or docs-only artifact.
• A maintainer can understand when to regenerate catalog/docs/CODEOWNERS and which files are generated.
• An external author can understand how to configure a copied/forked tap repo, scaffold a plugin, publish a raw JSON catalog, and tell users how to add that tap once DataDesigner supports it.
• Release docs explain how a plugin becomes discoverable and installable through the tap.
• Docs distinguish tap discovery from runtime entry-point discovery.
• make docs passes in strict mode.

Dependencies

• Depends on: #16, #18, #19, #20, #21, #22, #24.
• Related DataDesigner dependency marker: DD-CLI-TAPS.

Read the requirements in docs/taps.md, README.md, docs/authoring.md, docs/workflow.md, docs/releasing.md, and zensical.toml, along with the referenced dependency issues. Update the documented tap, authoring, workflow, and release contracts, add the tap page to navigation, and run make docs in strict mode; done means the acceptance criteria are covered and the documentation build passes.