Improve JSDoc comments with cross-references and clarifications (#254)

Key improvements:
- Add `{@link}` cross-references throughout the codebase to make API
  documentation navigable and help AI models understand relationships
- Use `{@link module!Export}` syntax for cross-module references
- Add backticks for inline code formatting (`ResizeObserver`, `App`, etc)
- Fix `PostMessageTransport` examples to show required two-argument form
- Clarify `onteardown` callback return type requirement
- Document React hooks cleanup behavior and Strict Mode considerations
- Add tool visibility example for `registerAppTool`
- Add CSP example for `registerAppResource`
- Fix notification direction documentation in `spec.types.ts`

TypeDoc config migrated from `typedoc.json` to `typedoc.config.mjs` to:
- Support custom `@description` block tag
- Add `intentionallyNotExported` for internal types
- Include `src/server/index.ts` entry point

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

typedoc.config.mjs

@@ -0,0 +1,30 @@
+import { OptionDefaults } from "typedoc";
+
+/** @type {Partial<import('typedoc').TypeDocOptions>} */
+const config = {
+  blockTags: [...OptionDefaults.blockTags, "@description"],
+  intentionallyNotExported: ["AppOptions"],
+  projectDocuments: ["docs/quickstart.md"],
+  entryPoints: [
+    "src/server/index.ts",
+    "src/app.ts",
+    "src/react/index.tsx",
+    "src/app-bridge.ts",
+    "src/message-transport.ts",
+    "src/types.ts",
+  ],
+  out: "docs/api",
+  gitRevision: "main",
+  excludePrivate: true,
+  excludeInternal: false,
+  categorizeByGroup: true,
+  navigationLinks: {
+    GitHub: "https://github.com/modelcontextprotocol/ext-apps",
+    Specification:
+      "https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/draft/apps.mdx",
+  },
+  readme: "README.md",
+  includeVersion: true,
+};
+
+export default config;