docs: migrate datasources from lb3 to lb4

Add steps to migrate LB3 datasources to LB4

docs/site/migration/datasources.md

@@ -6,8 +6,72 @@ sidebar: lb4_sidebar
 permalink: /doc/en/lb4/migration-datasources.html
 ---
 
-{% include note.html content="
-This is a placeholder page, the task of adding content is tracked by the
-following GitHub issue:
-[loopback-next#3946](https://github.com/strongloop/loopback-next/issues/3946).
-" %}
+## Overview
+
+LoopBack 3 datasources are compatible with LoopBack 4 datasources, so migrating
+datasources from LoopBack 3 to LoopBack 4 is simple.
+
+In LoopBack 3, all datasources are defined in the `server/datasources.json`
+file, whereas in LoopBack 4 each datasource is defined in its own file in the
+`src/datasources` folder. Each LoopBack 4 datasource has a configuration file
+(e.g. `db.datasource.config.ts`) and a class file (e.g. `db.datasource.ts`).
+
+## Migration Steps
+
+To migrate a datasource from LoopBack 3 to LoopBack 4, complete the following
+steps:
+
+1. In the root of your LoopBack 4 application, use the `lb4 datasource` command
+   to create a new datasource and enter the same datasource name as your
+   LoopBack 3 application's datasource (e.g. `db`):
+
+   ```
+   $ lb4 datasource
+   ? Datasource name: db
+   ```
+
+2. For the remaining prompts from the `lb4 datasource` command, use the defaults
+   (press Enter for each one) since these will be replaced in the next step:
+
+   ```
+   ? Select the connector for db: In-memory db (supported by StrongLoop)
+   ? window.localStorage key to use for persistence (browser only):
+   ? Full path to file for persistence (server only):
+   ```
+
+3. Replace the contents of the newly created
+   `src/datasources/{dataSource.dataSourceName}.datasource.config.ts` file in
+   your LoopBack 4 application with the datasource configuration from
+   `server/datasources.json` in your LoopBack 3 application.
+
+   {% include code-caption.html content="server/datasources.json" %}
+
+   ```json
+   {
+     "db": {
+       "name": "db",
+       "connector": "memory"
+     }
+   }
+   ```
+
+   {% include code-caption.html content="src/datasources/db.datasource.config.json" %}
+
+   ```json
+   {
+     "name": "db",
+     "connector": "memory"
+   }
+   ```
+
+4. Repeat steps 1-3 for each datasource you want to migrate.
+
+{% include note.html content="We are working on a CLI command `lb4 import-lb3-datasources` that will migrate datasources from a mounted LoopBack 3 application to a LoopBack 4 project automatically. See [GitHub issue #4346](https://github.com/strongloop/loopback-next/issues/4346) for more details." %}
+
+## Compatibility
+
+As mentioned before, LoopBack 3 datasources are compatible with LoopBack 4
+datasources. In both, a datasource is a connector instance that is used by
+`legacy-juggler-bridge`. For example, both a LoopBack 3 MySQL datasource and a
+LoopBack 4 MySQL datasource will use
+[`loopback-connector-mysql`](http://github.com/strongloop/loopback-connector-mysql).