Fix examples with Bazel@HEAD (#658)
This PR fixes multiple errors from the
Downstream:

Error with catch2 (mentioned in
https://github.com/bazelbuild/examples/issues/657) - Fix is updating its
version.
Error with loads in toolchains_llvm - Fix is updating its version.
Error with toolchains_protoc (name 'proto_library' is not defined)


Fix is removing toolchains_protoc since it was overriding an
incompatible version of toolchain.


Error with incompatible_use_toolchain_transition - Seemed to be
fixed with upgrade of toolchains_llvm.

Also, updated links for documentation which were not working.