Fully document ROBOTOLOGY_USES_CSHARP option

[traversaro]

The option ROBOTOLOGY_USES_CSHARP was requested in and added in #1346, but currently it is marked as "advanced", i.e. hidden until the user enables advanced CMake variables.

To stop marking it as hidden, the following steps are necessary:

• Add the option to CI
• Add the option to documentation
• Check if setup.* scripts needs to be modified

For an example of similar option that has been added to the docs and CI, see https://github.com/robotology/robotology-superbuild/pull/1217/files .

As nobody that I am collaborating with is using C#, I will not look into this, but contribution from the community in form of Pull Request will be welcome.