docs(site): fix build_docs npm script
docs(site): fix build_docs npm script

It should run build_global before build_es6_for_docs, not after. Also should run
build_cjs before tests2png.

Issues closed: #2111

RxJS-CN/RxJS-Docs-CN,SangKa/RxJS-Docs-CN,kwonoj/RxJS,benlesh/RxJS,martinsik/rxjs,ReactiveX/rxjs,trxcllnt/RxJS,staltz/RxJSNext,martinsik/rxjs,IgorMinar/RxJS,RxJS-CN/RxJS-Docs-CN,staltz/RxJSNext,benlesh/RxJS,ReactiveX/rxjs,jayphelps/rxjs,trxcllnt/RxJS,ReactiveX/rxjs,RxJS-CN/RxJS-Docs-CN,trxcllnt/RxJS,ReactiveX/rxjs,RxJS-CN/RxJS-Docs-CN,trxcllnt/RxJS,jayphelps/rxjs,staltz/RxJSNext,SangKa/RxJS-Docs-CN,martinsik/rxjs,jayphelps/rxjs,IgorMinar/RxJS,ReactiveX/RxJS,Brocco/RxJS,kwonoj/RxJS,kwonoj/RxJS,SangKa/RxJS-Docs-CN,staltz/RxJSNext,Brocco/RxJS,jayphelps/rxjs,SangKa/RxJS-Docs-CN,jayphelps/rxjs,martinsik/rxjs,Brocco/RxJS,kwonoj/RxJS,benlesh/RxJS,ReactiveX/RxJS,ReactiveX/RxJS,benlesh/RxJS,Brocco/RxJS,ReactiveX/RxJS