Documentation: document the new smbus console feature

This explains how to enable the SMBus console in coreboot
and its Kconfigs.

Change-Id: I50cafbbaaea133c9ea50131e455151287c96176a
Signed-off-by: Husni Faiz <ahamedhusni73@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67386
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <inforichland@gmail.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

Documentation/technotes/index.md

@@ -5,3 +5,4 @@
 * [Unit testing coreboot](2020-03-unit-testing-coreboot.md)
 * [Unit Test Code Coverage](2021-05-code-coverage.md)
 * [Address Sanitizer](asan.md)
+* [coreboot Consoles](console.md)