docs: kbuild/kconfig: reformat/cleanup

This document was using headings in an odd way, causing the sidebar to be
quite messy. I've adding new headings and turned some of the old headings
into description lists.

The indentation was a mix of spaces and tabs; I've turned them all into
4 spaces so it always reads correctly regardless of tab settings.

Also use ``...`` instead of `...`; the difference is that `` is meant
for "inline literals" (and renders in a monospace font) while ` is for
"interpreted text" (and renders with italics).

Also changed the title of the document to be more descriptive.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>