kconfig: docs: mention gconfig at top of kconfig.rst

kconfig: docs: mention gconfig at top of kconfig.rst

Jesse mentioned that gconfig is missing from the top of the
kconfig.rst file, so add it for completeness.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: lore.kernel.org/r/CAJFTR8QgYykuEq_AkODEWPUYXncKgRBHOncxT=ypZTQODkyarw@mail.gmail.com
Cc: Jesse Taube <mr.bossman075@gmail.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230704000120.8098-1-rdunlap@infradead.org