Works as so:
index.d2:
```d2
x: @x
```
x.d2:
```d2
label: meow
shape: circle
```
TODO:
- [ ] Correctly import into fields by handling the scenarios/steps overlay.
- And links.
- [ ] Test every kind of import.
- Boards with subboards are now selfcontained folders with index.svg/png as
the root board render.
- Boards that are only containers of other boards are not rendered. For example a
scenario with no modifications and only steps only has its steps
rendered.
- Boards with sibling boards of another type are rendered under a
subdirectory indicating their type to separate them. For example a
board with layers and scenarios has its layers rendered into subfolder
layers and scenarios into subfolder scenarios.
cc @berniexie see BoardContainer field on d2target.Board for the field
you were looking for to skip renders for PDFs too.
