Clarify doxygen landing page

[jenhicks]

On this page: https://simtk.org/api_docs/opensim/api_docs/

Change:

….then this documentation contains excessive information and you should check out the user API documentation.

to:

….then this documentation contains excessive information and you should check out the Scripting and Development (https://simtk-confluence.stanford.edu/display/OpenSim/Scripting+and+Development) documentation.

Make the “Scripting and Development” text clickable.