sdk: Add documentation warning of its development stage and drop CI for 3.1

This will give a reader a bit of context and fix the expectations when finding this documentation in the context of Oniro docs.

Related to eclipse/oniro-core/docs#147 (moved) and eclipse/oniro-core/docs#148 (moved)

We also drop CI for 3.1, for now, to lighten a bit the CI load while 3.1 is not the default version.