Split out persistent workspace into separate guide, remove design

The persistent workspace documentation is currently in the SDK usage document.
The usage document covers some very basic usage, whilst the persistent
workspace is a little bit more of an advanced topic. Split it out into its own
document and move both to guides.

Additionally the design document is still present for the persistent workspace
feature. This is now redundant since the design has been implemented. Move the
limited amount of pertanent information from the design document to the guide
and remove the design document to avoid confusion.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>