Update CMake quickstart with find_package + add CMake source build guide

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This PR updates the GoogleTest documentation as follows:

1. Update the CMake quickstart to show usage with find_package
2. Add a CMake source build guide for *nix single- and multi-config generators and Windows Visual Studio generators

Work was inspired by the conversation in #4681 and by previous experience building older (v1.10.0) versions of GoogleTest from source in Windows environments with both gtest.lib and gtestd.lib installed side-by-side linked against the shared C runtime. This is common in Windows development environments as there is more than one C runtime library and loading multiple (possibly ABI-incompatible) instances of the C runtime is generally discouraged.