Document how to suppress opening of the First Run Page

Background

For Large scale deployments there is a feature to suppress the first run page. This is useful if for example the user profile is wiped after reboots (schools, kiosk mode).

With the switch from XUL to WebExt the current documentation is out of date and needs to be updated.

See https://issues.adblockplus.org/ticket/6349#comment:11 for a summary of the current situtation.

What to do

Figure out a way to document this feature in the help center and add help center documentation accordingly.

Working doc of current draft: https://docs.google.com/document/d/1nK6sMGFH4KF2JT5wwuNA-l8_wwAWiH5sKeniYtkbgrU/edit#