[DirectX] Document DXContainer format

[llvm-beanz]

I should have written docs for this before I wrote the code. I need to do that now.

AC:
New docs under llvm/docs/DirectX documenting the DXIL container and appropriate container data.

• Document DXContainer structure
• Document PSV0 data format
• Document SFI0 format
• Document ISG0/OSG0 format