[Issue #649] Adding Plugins to Common Grants

[jcrichlake]

Summary

• Fixes [Plugin] List plugins on CommonGrants.org #649
• Time to review: 10 minutes

Changes proposed

What was added, updated, or removed in this PR.

This PR adds a plugin catalog page to the CommonGrants website and introduces bidirectional cross-linking between plugins and the custom fields they implement.

• Plugin catalog page — new /plugins index page listing all plugins from content/plugins/index.json, with filtering by language and system
• Plugin detail page — new /plugins/[slug] page rendered via PluginDetails.astro, showing plugin metadata and the custom fields it exposes as CustomFieldCard components
  linking to each field's detail page
• CustomFieldDetails.astro —Added a "Used By Plugins" section that filters plugins/index.json for plugins containing the
  current field and renders each as a PluginCatalogCard
• PluginDetails.astro — Added a CustomFieldCard grid (mirrors the pattern above)

Context for reviewers

Testing instructions, background context, more in-depth details of the implementation, and anything else you'd like to call out or ask reviewers. Explain how the changes were verified.

The cross-linking works as follows:

• On a custom field detail page (e.g. /custom-fields/agency), a "Used By Plugins" section appears only when at least one plugin includes that field. Each card links to
  /plugins/[slug].
• On a plugin detail page (e.g. /plugins/ts-cg-grants-gov), the custom fields section renders CustomFieldCard components, each linking to /custom-fields/[id].

Additional information

Screenshots, GIF demos, code examples or output to help show the changes working as expected.

Verification:

1. Navigate to /custom-fields/agency — confirm a "Used By Plugins" section appears with cards for both ts-cg-grants-gov and py-cg-grants-gov
2. Click a plugin card — confirm it routes to the correct /plugins/[slug] detail page
3. On the plugin detail page — confirm custom field cards appear and each links to /custom-fields/[id]
4. Navigate to /custom-fields/eligibilityCriteria (not used by either plugin) — confirm no "Used By Plugins" section appears
5. Navigate to /plugins — confirm the plugin catalog index renders correctly

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[bryan-thompsoncodes]

LGTM, played around with local website plugin views look sharp and the filtering works well even if there isn't much to filter yet 😄

[widal001]

Can we change this to match the name of the actual plugin? Or add a section that has that name?

So @common-grants/cg-grants-gov for the TypeScript plugin?

[widal001]

Sorry my comment meant to suggest that we make the label match the package name, or add a new attribute for package name, rather than changing the slug.

The reason we don't want to make the slug match the package name is that there might be name collisions between Python and TypeScript plugins, especially for third party TypeScript plugins that won't be prefixed with @common-grants/ since that's reserved for packages published to our npm org.

[widal001]

Also using @ and / symbols in the url can break the links (which is what happens with this package)

[widal001]

Woohoo!! This is a really exciting addition 🚀

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