Update reference to Diataxis framework and add alt-text

[maxrjones]

This PR adds alternative text to the image with Daniele Procinda's documentation system in content/tutorial-style-guide.md.

[rossbar]

When looking at this I realized that there is a newer link to this material that we should probably be using instead: https://diataxis.fr/. This also gives a name to the documentation system, Diataxis framework, which will make it easier to refer to.

[maxrjones]

When looking at this I realized that there is a newer link to this material that we should probably be using instead: https://diataxis.fr/. This also gives a name to the documentation system, Diataxis framework, which will make it easier to refer to.

OK, I can update accordingly but will not be able to do so right away.

[maxrjones]

When looking at this I realized that there is a newer link to this material that we should probably be using instead: https://diataxis.fr/. This also gives a name to the documentation system, Diataxis framework, which will make it easier to refer to.

I updated the reference and figure link in the latest commit.

[rossbar]

Thanks for this @meghanrjones - I took the liberty of slightly modifying the text to refer to the "Diataxis framework" by name rather than by author (attribution is maintained in the caption).