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>
1 file changed