docs(tree): add per-repo PROJECT_TREE snapshots + index them

Add auto-generated project-tree snapshots for the website, link, and
Android-app repos under docs/<repo>/PROJECT_TREE.md, and link them from
the README (adding a previously-missing android/ section). These files
are maintained going forward by the sync-project-tree CI workflow in each
source repo, which opens a PR here whenever the tracked layout changes.

Co-Authored-By: Claude <noreply@anthropic.com>

link/PROJECT_TREE.md

@@ -0,0 +1,44 @@
+# uo-link — Project Tree
+
+> **Auto-generated.** This file is maintained by the `sync-project-tree` CI workflow in
+> the [`RunicGateway/link`](https://gitea.whitlocktech.com/RunicGateway/link) repository, which
+> opens a pull request here whenever the tracked file layout on `main` changes. Do not edit
+> by hand — changes will be overwritten by the next sync.
+
+A snapshot of the tracked files in the repository (build output, dependencies, and other
+git-ignored paths are excluded).
+
+```text
+link-main/
+└── link/
+    ├── .gitea/
+    │   ├── ISSUE_TEMPLATE/
+    │   │   ├── bug_report.md
+    │   │   ├── config.yaml
+    │   │   └── feature_request.md
+    │   ├── workflows/
+    │   │   ├── release.yml
+    │   │   └── sonarqube.yml
+    │   └── PULL_REQUEST_TEMPLATE.md
+    ├── sidecar/
+    │   ├── src/
+    │   │   ├── config.rs
+    │   │   ├── main.rs
+    │   │   ├── rpc.rs
+    │   │   ├── shard.rs
+    │   │   ├── store.rs
+    │   │   └── web.rs
+    │   ├── .gitignore
+    │   ├── Cargo.lock
+    │   ├── Cargo.toml
+    │   ├── README.md
+    │   └── sidecar.toml.example
+    ├── .gitignore
+    ├── CODE_OF_CONDUCT.md
+    ├── CONTRIBUTING.md
+    ├── CONTRIBUTORS.md
+    ├── LICENSE.md
+    ├── README.md
+    ├── SECURITY.md
+    └── sonar-project.properties
+```