Update the expo documentation to include mention of this new object
type.

Signed-off-by: Simon Glass <sjg@chromium.org>

Add mention of persistent settings in the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>

This is related to standard boot, so put it under the same node. This may
simplify schema upstreaming later.

Mention themes in the documentation while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

This is mentioned in passing in the 'cedit' command. Its file format is
described under `expo`. But it would be better if it had its own entry
in the documentation.

Add a new 'cedit' entry with a few details about this feature.

Signed-off-by: Simon Glass <sjg@chromium.org>