Image update: Testframework moved to notebook repository.

[jachiang]

Fixes #118.

[jnewbery]

Looks good. Thanks @jachiang . A few points:

• I find the grey and blue backgrounds a little difficult to distinguish when they're small.
• it's confusing that the bitcoinops/bitcoin/tree/optech-taproot header is in blue, when that's supposed to denote jupyter notebooks.
• I know I keep saying this, but capitalization is inconsistent. "Jupyter Notebook Chapters" has every word capitalized, but "Notebook dependencies" only has the first word capitalized. Please pick a style and be consistent with it.

[jachiang]

Thanks @jnewbery, fixes have been applied.