[docs] Add DIA register instructions to Getting Started with Visual Studio page

Since Visual Studio 2017 the DIA libs are not registered by default, see:
https://docs.microsoft.com/en-us/visualstudio/extensibility/breaking-changes-2017?view=vs-2019#change-reduce-registry-impact
LLDB building instruction already specify registering these DLLs, required
both the LLVM PDB tests and LLDB build.

Differential Revision: https://reviews.llvm.org/D108811