[docs] Graduate VectorizationPlan.rst from proposal.

VPlan has become an integral part of the inner loop vectorizer pipeline
that has been actively developed over the previous years. Let's move
VectorizationPlan.rst from the proposal stage to bring the docs in line
and to avoid confusion when reading the docs.

Reviewed By: rengolin

Differential Revision: https://reviews.llvm.org/D157593