Merge branch '2023-03-17-improve-read-command-add-write-command' into next

To quote the author:
The first patch simplies do_read somewhat by making use of an existing
helper instead of parsing the dev_part string manually. As a bonus
(and my actual motivation), it now understands dev#partname syntax -
hard-coded partition numbers are so last decade.

I also need the symmetrical operation, being able to write to a named
raw partition, and fortunately it doesn't require that many lines of
code to implement that.

There's a very minor change in the error reporting due to using
cmdtp->name to generate the new messages, but I don't think "Error
reading blocks" offers much that "read error" doesn't.

New in v2: the last three patches add documentation, ensure CMD_WRITE
is set for sandbox and adds some basic test cases for the various ways
of accessing the partitions (by number, name, or as raw offset within
the whole disk).

v3: Add Simon's R-b to patches 2, 4, 5, fixup whitespace in patch 5.

I don't want to duplicate the documentation, but I can see the value
in 'write' having its own entry in the TOC, so I added a stub
write.rst that just refers to the read.rst, which then explicitly
documents both.