doc/dev/config: Document how to use :confval: directive for config options

doc/dev/config: Document how to use :confval: directive for config options

Add comprehensive guide for documenting configuration options using the
:confval: directive, including naming conventions and cross-referencing.

Previously, the documentation lacked guidance on using the :confval:
directive and the important distinction between regular config options
and mgr module options (which require the mgr/<module>/ namespace
prefix). This change provides detailed examples and best practices for
properly documenting and referencing both types of configuration options.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>