graphics/sdl2_gpu: disable DOCS option due to OOM condition in graphviz

For some reason graphviz now needs more than 20 GB to process one of
the figures in the documentation of this project.

Disable docs to avoid OOM conditions.

MFH: 2026Q1
(cherry picked from commit 1f4db82dbd52c18499fb8f7d222415324877dff3)