Documentation issue: "C toolchain configuration" contains extremely outdated information

Documentation URL: https://docs.bazel.build/versions/master/cc-toolchain-config-reference.html

At the bottom:

> This is a long list of features. The plan is to get rid of them once [Crosstool in Starlark](https://github.com/bazelbuild/bazel/issues/5380) is done.

The linked issue was closed mid-2018. If the protobuf CROSSTOOL file is no longer supported, it would be good to remove all documentation specific to it, ideally replacing with up-to-date documentation of the current mechanism.

It'd be really really nice to have a link to a few functioning and up-to-date toolchain configs - ideally with CI tests on bazel to ensure they stay in sync with the latest bazel version.

Start with the C toolchain configuration documentation URL and verify which material is specific to the protobuf CROSSTOOL file. Check the closed Crosstool in Starlark issue (#5380) and identify the current mechanism. Done means removing or replacing obsolete guidance and, where available, linking maintained toolchain configurations with CI coverage.

Written by the indexing model from the issue text.