Reconcile document site in 3.5.7 & 3.6.0

After ZOOKEEPER-3369 we generate document in apidocs/ instead of api/ before. Moreover, the document site cannot proper linked to apidocs in 3.6.0(but in 3.5.7, thought it is wrong in website branch, it is manually corrected in asf-site branch}}.

I propose we generate single aggregated document and place it under api/ folder, as well as bumping website for the update.