feat: add side blocks

[ShutdownRepo]

Hi 👋 it would be awesome to be able to have integrations put content on the left and right sides, under the table of contents for instance.

[addisonschultz]

Nice request! This isn't something possible just yet - but I'm wondering if you could maybe build a chrome extension that uses the GitBook API to do something similar?

Not sure exactly what you're aiming to build but that might be a quick workaround!

[ShutdownRepo]

The thing with the chrome extension is that the extension will be needed for the users that want to have that side block, while doing so from an internal integration allows the site to behave the same whatever the browser is