Merge pull request rails#54294 from takmar/create-database-in-plugin-documantation

[ci skip] Add instructions for setting up the testing database in plugin docs

Author: Edouard-chin

guides/source/plugins.md

@@ -68,6 +68,15 @@ spec.metadata["changelog_uri"] = "http://example.com"
 
 Then run the `bundle install` command.
 
+After that, set up your testing database by navigating to the `test/dummy` directory and running the following command:
+
+```bash
+$ cd test/dummy
+$ bin/rails db:create
+```
+
+Once the database is created, return to the plugin's root directory (`cd ../..`).
+
 Now you can run the tests using the `bin/test` command, and you should see:
 
 ```bash