docs/scripts/gdb: add necessary make scripts_gdb step

In order to debug the kernel successfully with gdb you need to run
'make scripts_gdb' nowadays.

This was changed with the following commit:

Commit 67274c08 ("scripts/gdb: delay generation of gdb
constants.py")

In order to have a complete guide for beginners this remark
should be added to the offial documentation.

Signed-off-by: Jakob Koschel <jkl820.git@gmail.com>
Link: https://lore.kernel.org/r/20230112-documentation-gdb-v2-1-292785c43dc9@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>