WIP: Update documentation to include starting postgre service after installation

What does this MR do?

While installing from source, running the second postgre setup step below before 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/or confirm if the service is up and running in case one forgets to start the service manually

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.

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