[DOC] removed deadlinks from the events/conferences; replaced with videoplay list from early Spark Summits#640

dmatrix wants to merge 12 commits intoapache:asf-siteapache/spark-website:asf-sitefrom

dmatrix:br_jsd_remove_more_deadlinks_community_pagedmatrix/spark-website:br_jsd_remove_more_deadlinks_community_pageCopy head branch name to clipboard