doc: develop: Add a general section on gdb usage

doc: develop: Add a general section on gdb usage

Mashed up from different sources linked below, including the now gone
Wiki and doc/README.arm-relocation file.  Tested on a custom board with
AT91 SAMA5D2 SoC and Segger J-Link Base adapter.  This is only generic
advice here, the usage is not board specific.  Some board docs have more
specific instructions on using gdb with a particular board.

Link: https://www.slideshare.net/slideshow/embedded-recipes-2019-introduction-to-jtag-debugging/177511981
Link: https://boundarydevices.com/debugging-using-segger-j-link-jtag/
Link: https://web.archive.org/web/20141224200032/http://www.denx.de/wiki/view/DULG/DebuggingUBoot
Link: https://web.archive.org/web/20141206064148/http://www.denx.de/wiki/view/DULG/GDBScripts1
Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexander Dahl <ada@thorsis.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>