docs: add a Flat Shifting page Flat shifting was listed as a supported feature on the Home page but linked to the external TunerStudio guide - there was no wiki page. Add a Flat Shifting page focused on the trigger and arming behaviour, which is the non-obvious part: it is rusEFI's torque-reduction feature, triggered by a pin that is often shared with launch control, with torqueReductionArmingRpm separating launch (below) from flat shift (above). Field names/comments are taken from firmware (integration/rusefi_config.txt). The exact reduction amount/method is deferred to TunerStudio; no numeric values are prescribed. Point the Home feature link at the new page. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>