Document gitlab-ctl generate-secrets command

The only documentation for the gitlab-ctl generate-secrets command is in the gitlab-ctl help command:

$ gitlab-ctl generate-secrets --help
Usage:
  gitlab-ctl generate-secrets -f|--file FILE

    f, --file=FILE                  Output secrets to file

More complete documentation should be added in:

• https://docs.gitlab.com/charts/installation/secrets.html

and or possibly:

• https://docs.gitlab.com/omnibus/maintenance/