docs: packing: move it to core-api book and adjust markups

The packing.txt file was misplaced, as docs should be part of
a documentation book, and not at the root dir.

So, move it to the core-api directory and add to its index.

Also, ensure that the file will be properly parsed and the bitmap
ascii artwork will use a monotonic font.

Fixes: 554aae35

 ("lib: Add support for generic packing operations")
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>