docker: fix compose self-hosting and add external Mongo/MinIO/Garage examples (#106)

* docker: update .env and docker-compose.yml for SMTP and new services

* docker: add external MinIO, MongoDB, and GarageHQ examples with setup instructions

* docs: update README with dotenv docker setup instructions and configuration redirect for external services

* docker: update service dependency condition for setup-themes in docker-compose.yml

* chore: update example URLs in .env for public services

examples/external-mongodb/README.md

@@ -0,0 +1,36 @@
+# External MongoDB
+
+MongoDB runs in a separate compose project. The Notesnook stack uses the root [`docker-compose.yml`](../../docker-compose.yml) with [`notesnook.override.yml`](notesnook.override.yml), which disables the embedded `notesnook-db` service and points at external Mongo. Embedded MinIO is unchanged.
+
+## Setup
+
+From the **repository root**:
+
+```bash
+# Merge into root .env
+cat examples/external-mongodb/.env.example >> .env
+# set NOTESNOOK_API_SECRET and other required root .env values
+
+# 1. MongoDB
+docker compose -f examples/external-mongodb/infra.compose.yml up -d
+
+# 2. Notesnook
+docker compose -f docker-compose.yml -f examples/external-mongodb/notesnook.override.yml up -d
+```
+
+## Environment (delta)
+
+See [`.env.example`](.env.example) — only `MONGODB_CONNECTION_STRING*` overrides are needed beyond the root `.env`.
+
+## Stop
+
+```bash
+docker compose -f docker-compose.yml -f examples/external-mongodb/notesnook.override.yml down
+docker compose -f examples/external-mongodb/infra.compose.yml down
+```
+
+## Notes
+
+- Replica set `rs0` is required; the infra healthcheck initializes it.
+- Host port `27017` is exposed for debugging.
+- If infra stops, `identity-server` and `notesnook-server` fail until MongoDB is back.