Update 2.3 -> 3.0 and hyperlinked "extensions" to the doc.
Update 2.3 -> 3.0 and hyperlinked "extensions" to the doc.

lanthaler/schemaorg,dbs/schemaorg,dbs/schemaorg,tfrancart/schemaorg,unor/schemaorg,dbs/schemaorg,tfrancart/schemaorg,gkellogg/schemaorg,gkellogg/schemaorg,schemaorg/schemaorg,vholland/schemaorg,schemaorg/schemaorg,lanthaler/schemaorg,vholland/schemaorg,gkellogg/schemaorg,schemaorg/schemaorg,vholland/schemaorg,gkellogg/schemaorg,unor/schemaorg,lanthaler/schemaorg,schemaorg/schemaorg,unor/schemaorg,tfrancart/schemaorg,tfrancart/schemaorg,vholland/schemaorg,dbs/schemaorg,schemaorg/schemaorg