doc: Document Zephyr's Kconfig configuration scheme

In particular, try to demystify Kconfig.defconfig files, and provide
guidelines on whether configuration settings should go in
BOARD_defconfig or Kconfig.defconfig.

The board porting section seems to be the most relevant one here, so put
the documentation there, with some "see also" links from elsewhere.
Things could be reorganized later if needed.

Give a general overview of visible and invisible Kconfig symbols as
well, as that ties in with the configuration scheme.

This is reverse-engineering on my part. The configuration scheme doesn't
seem to be documented anywhere prior.

Fixes: #7159

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>