Make a new kind of template available to `layout.html.st`

I'm not sure if this is possible currently, I couldn't work out how, anyway. Basically, I'd like a way to define something in `index.yaml` such that I can put it somewhere in the `layout.html.st` file. So `yst` defines `$nav$` and `$content$` based on what it reads in `index.yaml`. I'd like to make it such that there could be a third column, for instance. Or a sort of headline banner that runs across both nav and content and displays a different picture on each page.

Obviously, I could make it part of `$content$` and then do some CSS trickery to make it appear where I want, but it seems like a neater solution would be to just place it at the right place in the layout file.

Is this currently possible? Would implementing it be nontrivial?

Read how yst handles index.yaml and layout.html.st, starting with the existing $nav$ and $content$ values. Determine whether the current template mechanism can expose another named value; done means an additional region can be defined from the YAML data and placed explicitly in the layout.

Written by the indexing model from the issue text.