[DOC]Create documentation for _snapshot API

[kolchfa-aws]

What do you want to do?

• Request a change to existing documentation
• [x ] Add new documentation
• Report a technical problem with the documentation
• Other

POST /_snapshot/<repository>/_cleanup

PUT /_snapshot/<repository>/<snapshot>/_clone/<target_snapshot>

[hdhalter]

Hi Aria, this was requested in #1889. It is an API that was requested by the community. I closed that one in favor of this. Please take a look when you get a chance. Thanks!