Create cross references for directory structures

[mine-cetinkaya-rundel]

Put them in figure divs as well as a div that styles them (e.g., borders, margin/padding, indentation, etc.)

[cwickham]

I added this in the Projects chapter. E.g. in the source:

::: {#fig-dir-quarto .dir-str}

```{r}
file_create(dir, "_quarto.yml")
dir_tree(dir)
```

The folder `ds-project` is now a Quarto project because it contains `_quarto.yml`.

:::

And added the class .dir-str to quarto-tag.scss:

.dir-str {
  @extend .p-2;
  @extend .border;
  @extend .rounded
}

Which renders like this: