Add description for extra.scope option in the docs

[Bizordec]

Resolves #5964

[squidfunk]

Thanks for the PR! "Setting up site scope" is far too technical – if you look at the other setup guides, they always try to be use case centric, and "site scope" is too specific. We're currently restructuring the documentation and will be adding a guide called "Setting up a multi-site project" (working title), and site scope should be part of this guide ☺️

[Bizordec]

Agreed, your title is much better 😁
Will be waiting, thanks!

[squidfunk]

Closed because of #5964 (comment).