CRAVEX-Docs: Create and publish improved documentation - batch 2 - ScanCode Toolkit

[tsteenbe]

Rewrite the documentation of ScanCode toolkit (similar to how we did ScanCode.io https://scancodeio.readthedocs.io) according to Diátaxis framework.

[mjherzog]

Should this task be more specific to ScanCode Toolkit? The top-level text references scancode.io which seems to duplicate #43