docs: reference how to create Make variables in toolchain docs

The current docs on toolchains (https://docs.bazel.build/versions/master/toolchains.html) doesn't reference how to create Make variables nor that you can.

That information is on https://docs.bazel.build/versions/master/be/make-variables.html, however. A link to that would be nice with some summary text.

Start with the toolchains documentation at https://docs.bazel.build/versions/master/toolchains.html and review the related Make variables page at https://docs.bazel.build/versions/master/be/make-variables.html. Add a link and summary text explaining how to create Make variables; the work is done when toolchain readers can find and understand that information.

Written by the indexing model from the issue text.