Clean up and document fuzzers directory

[domenukk]

Currently, ./fuzzers is full of very similar fuzzers with hardly any differences. Also, it may not be clear what the differences are to the untrained eye. We should clean this up and document it.

Also, fuzzers like frida_libpng should be made more general (i.e.: not build libpng in build.rs)

[highonweb]

I am new to fuzzing and I went through the LibAFL docs and now going through all the modules and sub-modules. I can clean up and document each fuzzer as I learn about that modules used in each fuzzer.

[debrajrout]

i want to contribute in this section

[syheliel]

Is it OK to add description of different fuzzers(like baby_fuzzer_gramatron or baby_fuzzer_tokens) in docs/src?

[tokatoka]

docs/src is about the libafl book and I agree babyfuzzer's description should go there. but for others I think a single and improved README.md is enough(?)

cc @domenukk

[syheliel]

I have added some description for baby_fuzzer_* in #564

[shubhangi013]

Hey @tokatoka I am interested in working on this issue. I am a student developer from IIT BHU keen to contribute to AFL++.