webapp/billing: move FAQ to doc and some tweaks + linking to it
webapp/billing: move FAQ to doc and some tweaks + linking to it

DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,tscholl2/smc,tscholl2/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc