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>

Add the description of CLI commands to the generated index.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

Fixes: 44c5d7764bf4 ("doc: Add gpt command documentation")
Signed-off-by: Tom Fitzhenry <tom@tom-fitzhenry.me.uk>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

Adds a command called "gpt transpose" which will swap the order two
partition table entries in the GPT partition table (but leaves them
pointing to the same locations on disk).

This can be useful for swapping bootloaders in systems that use an A/B
partitioning scheme where the bootrom is hard coded to look for the
bootloader in a specific index in the GPT partition table.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>

Adds a command that can be used to modify the GPT partition table to
indicate which partitions should have the bootable flag set

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>

Adds an additional variable called gpt_partition_bootable that indicates
if the given partition is bootable or not.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>

Adds initial documentation for the gpt command

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>