Update plugin docs to recommend Data Designer CLI installs

[johnnygreco]

Context

Data Designer PR NVIDIA-NeMo/DataDesigner#618 adds the client-side data-designer plugin CLI workflow for discovering, inspecting, installing, and uninstalling plugin packages from the DataDesignerPlugins catalog.

Once that PR is merged, the plugin docs in this repo should recommend installing plugins through the Data Designer CLI as the primary path instead of leading with direct pip/uv package installation commands.

Scope

• Update the plugin user docs to show CLI-first installation guidance, for example data-designer plugin install <package-or-alias> where appropriate.

• Update the doc-creation / plugin authoring instructions so future plugin docs are generated or written with the CLI-first install recommendation.

• Add doc-creation guidance that code examples should follow the standard Data Designer import pattern:

  import data_designer.config as dd

• Keep direct pip/uv installation guidance where it is still useful as fallback, troubleshooting, or advanced-package-management guidance.

• Cross-link the catalog/package naming and alias behavior where that helps users choose the right install target.

Dependency

Wait for NVIDIA-NeMo/DataDesigner#618 to merge before making the CLI recommendation the default docs path.

Acceptance Criteria

• Plugin docs recommend Data Designer CLI installation once the CLI support has landed.
• Doc authoring instructions/templates produce or describe CLI-first installation sections for new plugins.
• Doc authoring instructions/templates tell authors to use import data_designer.config as dd in code examples.
• Any remaining direct package-manager install instructions are clearly positioned as fallback or advanced guidance.
• Relevant docs checks pass.