Fix Doxygen issues on Windows

[mbeyeler]

Using Doxygen on Windows produces an error: Here is the error message when type "doxygen.exe CARLsim.config.doxy" in command prompt

Generating index page...
explicit link request to 'ch1_getting_started' in layout file 'CARLsimLayout.xml' could not be resolved
...

This might be caused by "@ref" in the layout file:

<tab type="usergroup" visible="yes" title="1. Getting Started" url="@ref ch1_getting_started">

Needs to be replaced with:

<tab type="usergroup" visible="yes" title="1. Getting Started" url="\\ref ch1_getting_started">

However, the double-backslash \\ doesn't work in Linux... There we need a single backslash.
Maybe we can have two different layout and config files; one for Windows, one for Unix.

Fix.