Notebook should support markdown footnotes

The trick is that the footnote definition is separated from the text that refers to it by a blank, which the Dex notebook treats as a block boundary and ends up rendering in two separate markdown translation passes. Reproduction:

```
'Some text.[^explanation]

'[^explanation]: More text
```

does not generate a footnote, but should.

Start with the reproduction in the issue and inspect the Dex notebook's markdown block handling, especially the separate translation passes mentioned in the report. Verify the blank-separated footnote case and compare it with a footnote kept in one block. Done means the example generates the expected footnote.

Written by the indexing model from the issue text.