CMake: Improve install instructions

[Croydon]

This improves the CMake install instructions. The destination paths should not be hard-coded, but set as variables via GnuInstallDirs.

This is particularly important for package managers like Conan that require files to be installed in a custom directory.

PS: Merging this will make the Conan enet package patch free for the first time. Although other package managers like vcpkg will still carry additional CMake patches for now.

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• Closes Add install to CMakeLists.txt #184
• Closes Enable MSVC build and install #104
• Closes CMake improvements #147 (I meant to specify this in CMake: Enable shared builds #242 already, but messed up the keyword, closing vs closes)