Add Getting Started guide for main desktop platforms

Create main page that links to seperate page for getting editor started from git clone to build and run:
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-Windows
-Mac
-Linux
-FreeBSD

Include main modules and 32bit and 64bit versions

Document how to link and include module of user choice.
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Currently supported modules are:
* SDL2 ( 32bit and 64 bit)
*  Raylib ( 64 Bit only)  