docs: move COMPILE.TXT to docs/, and split it into separate files for *nix & Windows

docs/COMPILE.md

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+To compile Keystone on Mac OS X, Linux, BSD, Solaris and all kind of nix OS,
+see [COMPILE-NIX.md](COMPILE-NIX.md)
+
+To compile Keystone on Windows, see [COMPILE-WINDOWS.md](COMPILE-WINDOWS.md)
+
+Then learn more on how to code your own tools with our samples.
+
+ - For C sample code, see code in directory samples/sample*.c
+ - For Python sample code, see code in directory bindings/python/sample*.py
+ - For samples of other bindings, look into directories bindings/<language>/