Promote ScanCode.io on the ScanCode-toolkit repo and documentation

[tdruez]

We should add some references to ScanCode.io app in the toolkit README and documentation.

• Homepage at https://scancode.io
• Code at https://github.com/nexB/scancode.io
• Documentation at https://scancodeio.readthedocs.org/

[arushi-mittal]

Hi, I'd like to work on this if it's still open. Please assign it to me!

[purna135]

Hi @tdruez can I work on this ?

[suhasml]

Hello, if this issue is still open, I would like to work on it.

[the-illuminatus]

has the issue been closed?

[pombredanne]

you do not need to ask permission :) If you want to work on it, say so and submit a PR.

[Avan2021]

I am working on it

[Avan2021]

Hey @pombredanne, I have updated the documentation. Please review it and tell me if I need to make any changes.
#4108