Docs: Generate attribution with custom template tutorial

[tdruez]

Following the implementation of attribution generation in #684
It'll be a good candidate for a new tutorial going through the process of crafting a custom template and providing it to the project using the .scancode configuration directory.

[tdruez]

Base instructions for custom attribution:

1. Make your changes to the default template https://github.com/nexB/scancode.io/blob/main/scanpipe/templates/scanpipe/attribution.html
2. Paste your template content directly in your project settings UI
3. Use the download: “Attribution” button from the project details view