docs: reorganize into design, security, api, ops and dev; drop stale designs; refresh the diagrams [CI]

- Group docs by category with an index (docs/README.md); fix every link,
  code comment and the protocol conformance test that read docs/protocol.md.
- access.md and security.md rewritten to what is implemented: removed the
  unadopted member-proof chain, roles and recovery key that do not exist,
  the old implementation order and the budgets; gaps now carry a status.
- enrollment, deployment, api: the install flow is the static, hash-checked
  script with environment variables; no default admin.
- README and architecture: new topology and update-flow diagrams (operator
  passkey, CI signing, registry feed, auth and trust modules).

backend/fleet/src/Fleet.Hosting/TrustAnchor.cs

@@ -25,7 +25,7 @@ public sealed partial class TrustFileContext : JsonSerializerContext;
 /// <summary>
 /// 节点的信任根：安装命令里带来的创世指纹和属主，在注册时固定到本地，之后 Backend 改不了。
 /// 操作员集合的新版本随期望状态下发，节点只接受从固定的创世出发、签名完整、版本更新的链。
-/// 属主只是被固定和保存：节点现在没有 extension，等做 extension 时由节点本地强制（见 docs/access.md 第 8 节）。
+/// 属主只是被固定和保存：节点现在没有 extension，等做 extension 时由节点本地强制（见 docs/security/access.md 第 9 节）。
 /// </summary>
 public sealed class TrustAnchor : IServerVerifier
 {