feat(showcase): build the static gallery generator

[salazarsebas]

Part of #240

Background

Once the example catalog has structured metadata (sibling sub-issue), nothing renders it. The showcase needs to be a static, build-time-generated set of pages, no backend, no database, per docs/strategy/06-product-strategy.md, so it can be hosted alongside the documentation site at zero incremental infrastructure cost.

Objective

Build a generator that consumes the example catalog metadata and produces a browsable gallery: an index page with filtering by category/maturity, and one detail page per example.

References

• Product spec: docs/strategy/06-product-strategy.md (showcase section)
• Design tokens/visual system this generator's output must consume: docs/strategy/09-design-strategy.md and the design-system epic's shared tokens package (dependency: this issue should consume, not duplicate, the tokens package once it exists)
• mdBook's plugin/preprocessor model, if the showcase is implemented as an mdBook section rather than a fully separate generator: https://rust-lang.github.io/mdBook/format/configuration/preprocessors.html

In scope

1. A decision, stated in the PR, on implementation approach: an mdBook preprocessor that generates showcase pages inline with the docs site, or a small standalone static-site generator (Rust or Node) whose output is placed alongside the mdBook build. Either is acceptable; the constraint is zero backend/database and one build step.
2. Index page listing all cataloged examples with category and maturity filters (client-side filtering is sufficient, no server required).
3. One detail page per example: name, category, maturity badge, cougr_features list, description (sourced from the example's own README.md), and a link to its source directory on GitHub.
4. Generator runs as part of the docs site's existing CI build (from the docs-site epic), not as a separate manual step.

Out of scope

• The preview/screenshot images themselves (sibling sub-issue provides the source images; this issue only needs to render an <img> where one exists and gracefully omit it where one doesn't).
• The "Cougr Verified" badge logic (sibling sub-issue); this issue only needs to render whatever verification status the catalog data provides.

Definition of done

• Gallery index renders all cataloged examples with working category/maturity filters.
• Each example's detail page pulls its description from the example's own README.md without manual duplication.
• Generator is wired into the docs site's CI build.

[directorfloo]

@directorfloo has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would love to work on this issue professionally, please assign it to me

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[victor-134]

@victor-134 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Kindly assign to me sir 😭🤲

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[Blessed-Femi]

@Blessed-Femi has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I will love to work on this

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[Muyideen-js]

@Muyideen-js has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi, i can fix this

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[drips-wave]

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