NEW: Adding more user documentation with a FAQ

Who's the target audience for these docs?

You've mentioned a `SubsiteID` column in the database, but if these docs need to explain the way subsites works technically, more explanation on things like `augmentSQL` in `SiteTreeSubsites` etc might be useful, and how it hooks into existing requests (e.g. via domain or ?SubsiteID=5 in URL) to serve a subsite page instead of a "normal" page request.

Also, what do you mean by "redirect url"?