Update documentation to check postgre status

What does this MR do?

While installing from source, executing the step below without first ensuring that the postgre service is started will result in the error below

2. Create a database user for GitLab:

   root@f04ec368f96e:/tmp# sudo -u postgres psql -d template1 -c "CREATE USER git CREATEDB;"
   psql: could not connect to server: No such file or directory
    Is the server running locally and accepting
         connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

I propose to include in the documentation a process to start the postgre database service and confirm if the service is up and running.

This can be done using these two commands

   sudo service postgresql start
   sudo service postgresql status

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.