Update some SDK docs from MVP (#3212)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3212

doc changes including
1. Remove instruction for Buck because we're moving away from it and just use CMake now and future;
2. Remove Coming soon for the realized feature;
3. Formatting.

Reviewed By: Jack-Khuu

Differential Revision: D56433016

fbshipit-source-id: fffa283b4a04438866d84765a65377dcf8a88837