gmxapi CMake documentation not included in build docs
gmxapi has several CMake variables, logic, and caveats that should probably be in the dev-manual/build-system document
Note GMXAPI CMake variables and build targets.
`build-system.rst` is a good place to note CMake variables and targets,
but the `GMXAPI` option, and the `Gromacs::gmxapi`, `gmxapi-cppdocs`,
and `gmxapi-cppdocs-dev` CMake targets, were not mentioned there.
Also, changes the names of the targets to use hyphens instead of
underscores for word separators.