Add vitepress docs

docs/app/topics.md

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+# Topics
+
+The topics system provides a reactive way to track and count hashtags across all events in the repository. It automatically updates as new events arrive or are removed.
+
+```typescript
+import {topics} from '@welshman/app'
+
+// In a Svelte component
+<script>
+  // Reactive list of all topics with counts
+  $: topicList = $topics
+    .sort((a, b) => b.count - a.count)
+    .slice(0, 20)
+</script>
+
+<div class="topics">
+  {#each topicList as {name, count}}
+    <a href="/t/{name}">
+      #{name}
+      <span class="count">({count})</span>
+    </a>
+  {/each}
+</div>
+```
+
+The store:
+- Updates automatically with new events
+- Maintains topic counts
+- Is throttled to prevent excess updates
+- Is case-insensitive
+- Integrates with the repository
+
+Think of it as a live tag cloud that stays in sync with your local event cache.
+
+This is commonly used for:
+- Tag clouds
+- Topic discovery
+- Content organization
+- Trending topics