docs(postgres): avoid using the admin user. (#4975)

docs: Update the documentation to avoid using the postgres user.

In some environments it is not possible to get access to the postgres
user.

Co-authored-by: Silvan <silvan.reusser@gmail.com>

docs/docs/self-hosting/manage/database/_postgres.mdx

@@ -35,4 +35,22 @@ Database:
         RootCert:
         Cert:
         Key:
-```
+```
+
+The admin user is only needed for the installation step but can also be avoided
+by preparing the necessary requirements:
+
+* the zitadel user
+* the zitadel database
+* granting required permissions to the zitadel user
+
+```sql
+CREATE ROLE zitadel LOGIN;
+CREATE DATABASE zitadel;
+GRANT CONNECT, CREATE ON DATABASE zitadel TO zitadel;
+```
+
+Don't forget to adjust pg_hba.conf and set a password for the zitadel user.
+
+With the setup done, follow the [phases guide](/docs/self-hosting/manage/updating_scaling#separating-init-and-setup-from-the-runtime)
+to run the init and then setup phase to get all necessary tables and data set up.