Documentation: split devicetree driver hookup into separate page

Move the devicetree driver example into a separate page under the
drivers category, and link to it from both the devicetree page and
the drivers index page. This makes more sense from a grouping
perspective and makes the info easier to find.

Change-Id: Ic3ca80b93a0020737c7ccb5313a0877172022e1a
Signed-off-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67762
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

Documentation/drivers/index.md

@@ -4,6 +4,10 @@ The drivers can be found in `src/drivers`. They are intended for onboard
 and plugin devices, significantly reducing integration complexity and
 they allow to easily reuse existing code across platforms.
 
+For details on how to connect device drivers to a mainboard, see [Driver Devicetree Entries](dt_entries.md).
+
+Some of the drivers currently available include:
+
 * [Intel DPTF](dptf.md)
 * [IPMI KCS](ipmi_kcs.md)
 * [SMMSTORE](smmstore.md)