docs: Add migration guide for hosted managed databases (sqlc-dev#3417)

docs/_templates/breadcrumbs.html

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docs/guides/migrating-off-hosted-managed-databases.md

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+# Migrating off hosted managed databases
+
+Starting in sqlc 1.27.0, [managed databases](../docs/managed-databases.md) will require a database server URI in the configuration file.
+
+This guide walks you through migrating to a locally running database server.
+
+## Run a database server locally
+
+There are many options for running a database server locally, but this guide
+will use [Docker Compose](https://docs.docker.com/compose/), as it can support
+both MySQL and PostgreSQL.
+
+If you're using macOS and PostgreSQL, [Postgres.app](https://postgresapp.com/) is also a good option.
+
+For MySQL, create a `docker-compose.yml` file with the following contents:
+
+```yaml
+version: "3.8"
+services:
+  mysql:
+    image: "mysql/mysql-server:8.0"
+    ports:
+      - "3306:3306"
+    restart: always
+    environment:
+      MYSQL_DATABASE: dinotest
+      MYSQL_ROOT_PASSWORD: mysecretpassword
+      MYSQL_ROOT_HOST: '%'
+```
+
+For PostgreSQL, create a `docker-compose.yml` file with the following contents:
+
+```yaml
+version: "3.8"
+services:
+  postgresql:
+    image: "postgres:16"
+    ports:
+      - "5432:5432"
+    restart: always
+    environment:
+      POSTGRES_DB: postgres
+      POSTGRES_PASSWORD: mysecretpassword
+      POSTGRES_USER: postgres
+```
+
+```sh
+docker compose up -d
+```
+
+## Upgrade sqlc
+
+You must be running sqlc v1.27.0 or greater to have access to the `servers`
+configuration.
+
+## Add servers to configuration
+
+```diff
+version: '2'
+cloud:
+  project: '<PROJECT_ID>'
++ servers:
++ - name: mysql
++   uri: mysql://localhost:3306
++ - name: postgres
++   uri: postgres://localhost:5432/postgres?sslmode=disable
+```
+
+## Re-generate the code
+
+Run `sqlc generate`. A database with the `sqlc_managed_` prefix will be automatically created and used for query analysis.