Adds documentation for stage button
Adds documentation for stage button

Includes missing documentation for 'Play all manual' builds a single
stage.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/28741

stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,stoplightio/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq