Title: Outdated Instructions in bazel/EXTERNAL_DEPS.md

Description:
It looks like the documentation in bazel/EXTERNAL_DEPS.md is a little outdated. Specifically, for external dependencies that use CMake, we're told to add a cmake_external rule to bazel/foreign_cc/BUILD but I don't see any references to cmake_external in the codebase. It looks like envoy_cmake (which calls cmake) is used instead. Should this be updated?