Provide top level CMake configuration and clear build instructions

Project exposes multiple things:
* PTI library
* Tools (such as unittrace)
* Samples

However, it's unclear how to build some of the above. Product instructs only to build samples leaving instructions for PTI library and tools behind the scene:
https://github.com/intel/pti-gpu/blob/a928b93abb9b06d2b6e78c22bb6446c3437efb3f/README.md?plain=1#L122

Can you, please, consider to make the following changes:
1. Flatten CMake configuration by exposing top level CMakeLists.txt file which will allow to build everything at once?
1. Adjust documentation by:
1. Describing this build entry point
1. Describing available build options to select build targets
2. Describing dependencies paying attention to any which are closed source dependencies

CC: @mschilling0, @jfedorov, @frenchwr