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>

docs/dev-guide/book.toml

@@ -0,0 +1,23 @@
+[book]
+title = "PiCloud Developer Guide"
+description = "How to build and run serverless apps on PiCloud — write Rhai scripts, get HTTP endpoints."
+authors = ["PiCloud"]
+language = "en"
+src = "src"
+
+[output.html]
+default-theme = "navy"
+preferred-dark-theme = "navy"
+smart-punctuation = true
+git-repository-url = ""
+edit-url-template = ""
+
+[output.html.fold]
+enable = true
+level = 1
+
+[output.html.search]
+enable = true
+limit-results = 30
+use-boolean-and = true
+boost-title = 2