doc: cmd: gpt: add information on type partition
Add information on type partition, copied from README.gpt.
I also correct issue for gpt_parts variable and add example of
"gpt read" usage.
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
authored by