Fixes links in the Vulkan back-end doc. Also includes a link to the
Vulkan back-end docs on the main page. Also updates the building code
section to mention VS2019 and make the win toolchain varable set more
prominent.

Bug: angleproject:1944
Change-Id: I8e8f0775daa3643afaa1ddd44429fa7d8e77b19b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1846014Reviewed-by: Jamie Madill <jmadill@chromium.org>