Some improvements to the notebook generator.

[penelopeysm]

1. it should skip Julia blocks marked with eval: false (easy enough, when parsing just convert to a Markdown cell)

```{julia}
#| eval: false
#| output: false
using Pkg
Pkg.add("Turing")
```

2. the callout block replacement regex is too sensitive, it doesn't capture things like ::: {.callout-note title="Note on prerequisites"}

Neither of these are particularly urgent, but they're also pretty simple fixes.