Fixing confusing documentation formatting

See the general Documentation guidelines: https://docs.gitlab.com/ee/development/documentation/.

What does this MR do?

Fixes the formatting in the "Moving the home directory for a user"

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page.
• Change the header tag to h5, #####.

Moving docs to a new location?

See the guidelines: https://docs.gitlab.com/ee/development/documentation/#changing-document-location.

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.

Appears to be caused by missing indentation on code blocks. Looks fine in markdown preview, but not on documentation web page, which apparently uses the Kramdown engine.

Updated doc section:

1. Stop GitLab

   gitlab-ctl stop

2. Stop the runit server

   # Using systemctl (Debian => 9 - Stretch):
   sudo systemctl stop gitlab-runsvdir
   
   # Using upstart (Ubuntu <= 14.04):
   sudo initctl stop gitlab-runsvdir
   
   # Using systemd (CentOS, Ubuntu >= 16.04):
   systemctl stop gitlab-runsvdir.service

3. Change the home directory. If you had existing data you will need to manually copy/rsync it to these new locations.

   usermod -d /path/to/home USER

4. Change the configuration setting in your gitlab.rb

   user['home'] = "/var/opt/custom-gitlab"

5. Start the runit server

   # Using systemctl (Debian => 9 - Stretch):
   sudo systemctl start gitlab-runsvdir
   
   # Using upstart (Ubuntu <= 14.04):
   sudo initctl start gitlab-runsvdir
   
   # Using systemd (CentOS, Ubuntu >= 16.04):
   systemctl start gitlab-runsvdir.service

6. Run a reconfigure

   gitlab-ctl reconfigure