kconfig: doc: Split up symbol reference by path

Use --modules to split the Kconfig reference up by where symbols are
defined. See the argparse docstring for --modules in
doc/scripts/genrest.py.

Not sure what the best way to split things up is, so feedback would be
appreciated. I just pulled out some top-level directories. It could
always be tweaked later.

If a symbol is defined in more than one module (by being defined in
multiple locations), it appears on multiple index pages.

To build the documentation, do

    $ mkdir doc/b && cd doc/b
    $ cmake -GNinja ..
    $ ninja htmldocs
    (output in html/reference/kconfig/index.html)

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>