[drake_bazel_external] Commercial solvers

[jwnimmer-tri]

The drake_bazel_external documentation doesn't give any hint about how to turn on the commercial solvers for mathematical programs (SNOPT, MOSEK, Gurobi) when using Drake as a Bazel external. We should document it.

[jwnimmer-tri]

The easy answer to recommend is that in the downstream tools/bazel.rc file within their project, to add something like this:

# Enable Gurobi by default.
build --define=WITH_GUROBI=ON

Maybe just some commented-out lines in our example drake_bazel_external/tools/bazel.rc would be good enough.

[pvarin]

It might also make sense to mention that we need to add the license file environment variable to allow tests to work

test --test_env=GRB_LICENSE_FILE