CMake: flamegpu_add_library

[ptheywood]

Document the existance of flamegpu_add_library introduced in FLAMEGPU/FLAMEGPU2#1062.

There's not currently a great home for this within the userguide, so probably need to introduce a project or cmake type page?, which could also document the add_executable version.

tldr;

• Use add executable if you just want a single binary
• Use add_library if you want a common base which interacts with flamegpu, that can then be linked against by one or more executables, i.e. for a simulator and a test suite.