[AFL++] Documentation: docs/testing/fuzzing.md (local, CI, triage)

[vyavdoshenko]

Scope

• Developer-facing documentation covering:
  • Installing AFL++, building with -DDFLY_FUZZ=ON.
  • Running single and master–replica targets locally.
  • Adding new targets, dictionaries, and seeds.
  • Reproducing crashes from artifacts; how CI jobs behave; where to find artifacts.

Tasks

• Create fuzzing.md with step-by-step guides.
• Keep examples minimal and copy-paste friendly.