docs: extend docs

docs/components/core.md

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+# Core
+
+The `Core` resource configures the essential Supabase services:
+
+- PostgREST
+- Auth
+
+and it manages the initial DB migrations that are not done by the services themselves and are part of the [supabase/postgres](https://github.com/supabase/postgres) repository.
+
+To deploy a basic `Core` instance, you can use the following snippet as a 'template':
+
+```yaml title="Basic 'Core' example" linenums="1"
+--8<-- "config/samples/supabase_v1alpha1_core.yaml"
+```