Rewrite ScanCode docs according to Diátaxis framework

[tsteenbe]

Rewrite the documentation of ScanCode docs (see https://scancodeio.readthedocs.io) according to Diátaxis framework to make the AboutCode docs better. We start with ScanCode as that one of the most used AboutCode projects and @tsteenbe who working on this issue is year long ScanCode user.

[mjherzog]

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