docs(website): add User Stories and Use Cases collage page (#18282)

Adds a new top-of-sidebar docs page at /docs/user-stories that is a
masonry-style collage of 99 real user stories sourced from X/Twitter,
GitHub issues/PRs, Reddit, Hacker News, YouTube, blogs (Medium, Substack,
dev.to), podcasts, LinkedIn, GitHub Gists, and Product Hunt.

Every tile links to the original post/issue/video/gist where someone
described a specific use case: personal assistants, dev workflows,
trading bots, research briefs, family WhatsApp agents, Kubernetes
deployments, legal-domain self-hosted setups, and more.

- docs/user-stories.mdx: MDX entry mounting the collage component
- src/components/UserStoriesCollage: React component with category +
  source filters, CSS-columns masonry layout, per-category accent colors
- src/data/userStories.json: source-of-truth dataset (force-added; the
  root .gitignore's unanchored 'data/' rule would otherwise swallow it,
  same reason skills.json is explicitly listed in website/.gitignore)
- sidebars.ts: link added at the top of the docs sidebar

website/docs/user-stories.mdx

@@ -0,0 +1,10 @@
+---
+title: User Stories & Use Cases
+description: Real stories from the Hermes Agent community — what people are actually building, scraped from X, GitHub, Reddit, Hacker News, YouTube, blogs, and podcasts.
+hide_title: true
+hide_table_of_contents: true
+---
+
+import UserStoriesCollage from '@site/src/components/UserStoriesCollage';
+
+<UserStoriesCollage />