feat(docs): generate plugin docs pages

[eric-tramel]

Summary

• Add ddp plugin-docs to assemble docs/plugins/ from plugins/*/docs/ plus plugin package metadata.
• Replace the generated plugin table with generated plugin landing pages and Zensical navigation entries.
• Scaffold docs/index.md for new plugins and add template plugin docs as the reference source.
• Refine the plugin browser card UI and template plugin installation docs.
• Add a transparent generated Data Designer Plugins icon and use it as the docs logo and favicon.
• Remove the old legacy plugin index command and Makefile alias so plugin docs are the only docs-generation path.
• Wire make docs, make docs-server, make check, and docs CI paths into the plugin docs flow.

Validation

• uv run pytest devtools/ddp/tests -q
• uv run ruff check .
• uv run ruff format --check .
• make check
• make docs
• make all

Notes

This stacks on PR #3 (codex/zensical-docs) because it replaces the plugin table introduced there with per-plugin documentation pages.