doc/manual: Add guideline for configuration files

doc/manual: Add guideline for configuration files

The goal of this guideline is to have consistent and well-structured
configurations files. The focus of this patch is on filenames and
directory structure. A guideline for the content of the files should be
included in a subsequent patch.

This patch addresses a long outstanding task listed in 'Pending and Open
Tasks'.

Change-Id: Ib32dd8b9ed15c3f647cd8d74cfc79edf0e79a3df
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8854
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>