document the fact that gurobi and mosek are not available in the binary builds

[RussTedrake]

In responding to this question:
https://stackoverflow.com/questions/62587294/nightly-build-doesnt-acquire-mosek-license/62613736#62613736
I went to look for the place in the documentation, or an open issue, that I could cite. I didn't find one.

https://drake.mit.edu/doxygen_cxx/group__solvers.html says SNOPT is provided in the binary release, but I didn't see anywhere we say that Gurobi and Mosek are not.

Is it written somewhere? If not, can we add it?
cc @hongkai-dai

[hongkai-dai]

No I don't think it is written anywhere. We should probably add it in https://drake.mit.edu/from_binary.html

[jwnimmer-tri]

I've added this to the #13539 Epic.

[jamiesnape]

As far as the Stack Overflow question goes, I guess pydrake.solvers.mosek.MosekSolver.AcquireLicense().is_valid() returning False is technically correct, but maybe it is not the most user friendly outcome?

[ToffeeAlbina-TRI]

@RussTedrake this is captured under umbrella #13539 which is on deck on the build system, continuous integration, distribution board, so I'm removing it from the improve pydrake documentation user experience board.

[jwnimmer-tri]

The place to mention this is probably the four pages linked from https://drake.mit.edu/installation.html (at the "Stable" links), as well as the https://drake.mit.edu/doxygen_cxx/group__solvers.html overview.