Preprocessor documentation

[chrmod]

We need a proper high-level description of preprocessors and their role in the adblocker library.
Comments like https://github.com/ghostery/adblocker/pull/4016/files#diff-6bbed6f2ce82ce3e568015ed0042560e15d069e1ad2735edbba1dc1046780de6R445 are not easy to understand from the newcomer perspective.

Need to think where is the best place to describe the setup and its details.

We can start with something like Preprocessors are a layer on top of FiltersEngine to exclude some filters at runtime based on conditions from the environment but they don't change which filters are in the FiltersEngine