Create a multisite guide

[gchtr]

Ticket: #1434

There are some limitations when getting posts from other pages in a multisite network. Specifically, when you fetch posts from other blogs, you’ll have to prepare the data in advance.

The linked issue has a very good example for this, that I’ve run into myself, too. I think we should document that in a new Multisite Guide.

There’s also some functionality in the Timber\Site class that we could document from a different perspective in the new guide.