[sdk][docs] Document the role of SSH when working with a target device.

This documentation is intentionally minimal for now, as more related documentation will soon be added, which will likely require moving some information around.

Test: verified that docs appeared in SDK archives.
Bug: DX-546
Bug: DX-732
Change-Id: I6c72f31b2b6cc32cb716eff3e3b8906181f0d90c
2 files changed