docs: Bring yocto-build-system assets back to git

We have moved the artefacts that were forcing us to use LFS due to their
size (blueprints). The only remaining one is `oniro-arch.png` which we
can just bring it back in git for now (54K).

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>