* docs - add content for backup storage plugin api

* some edits requested by david

* comments from mel; remove diagram placeholder

* address some requested edits from karen

* gpbackup creates the local directory

* include restore more

* remove bold styling from command

* clarify streaming backup

* address some review comments

* add backup/restore plugin api cmds to subnav

* remove the on-page toc entries for cmds

* wrap up the edits from karen