[VPlan] Document current status. (#85689)

Update VectorizationPlan.rst to include a section about the current
status of VPlan and its use in LoopVectorize, based on "VPlan: Status
Update and Future Roadmap", LLVM Developers’ Meeting 2023,
https://www.youtube.com/watch?v=SzGP4PgMuLE

PR: https://github.com/llvm/llvm-project/pull/85689