doc(parallelGW/asyc): doc custom extensions

related to #CAM-2217
doc(userguide): add note on async continuation for parallel GW

related to #CAM-2217
fix(docs/bpmn20): fix parallel gateway exclusive/async links

realted to #CAM-2217
chore(download): add 7.2.0-alpha1 to download page
fix(download): adjust 7.2.0-alpha1 release date
fix(download) release notes url