docs: add comprehensive developer guide (mdBook)

Add a browsable mdBook developer guide under docs/dev-guide/ covering the
full PiCloud v1.1.9 surface for developers building on the platform:

- Guide: introduction, 10-minute quickstart, core concepts, writing scripts
- Tutorials: 5 end-to-end example apps (URL shortener, webhook receiver,
  TODO API with auth, scheduled report, file-upload service) + feature matrix
- SDK reference: kv/docs/files, pubsub/queue, http, email, users, secrets,
  invoke/retry/dead_letters, stdlib, ctx & events
- HTTP API reference: every /api/v1 endpoint, auth, and capability
- CLI reference: the `pic` client + the server admin recovery command
- Config: every PICLOUD_* env var, capabilities & roles
- Deployment: Docker Compose, bare binary, Caddy/TLS, production checklist
- Operations: security, best practices, troubleshooting

Every example was built and run against a live instance; SDK signatures,
endpoint paths, and env vars were verified against source. Build the site
with `mdbook serve docs/dev-guide`. A pointer is added to README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -28,6 +28,20 @@ cargo check --workspace
 cargo run -p picloud
 ```
 
+## Documentation
+
+The **Developer Guide** in [`docs/dev-guide/`](docs/dev-guide/) is the place to start using PiCloud —
+quickstart, core concepts, full SDK / HTTP-API / CLI reference, five end-to-end example apps, and
+deployment + security guides. Build and read it locally with [mdBook](https://rust-lang.github.io/mdBook/):
+
+```sh
+cargo install mdbook          # once
+mdbook serve docs/dev-guide   # then open http://localhost:3000
+```
+
+Architecture and contributor notes live alongside it in [`docs/`](docs/) (`sdk-shape.md`,
+`stdlib-reference.md`, `versioning.md`, …) and in [`serverless_cloud_blueprint.md`](serverless_cloud_blueprint.md).
+
 ## Repository Layout
 
 ```