Update reference to Diataxis framework and add alt-text (#97)

* Add alt-text to image in content/tutorial-style-guide.md

* Update documentation system link and reference

* Minor nit: refer to diataxis by name.

Co-authored-by: Ross Barnowski <[email protected]>

Author: Meghan Jones

content/tutorial-style-guide.md

@@ -13,8 +13,8 @@ kernelspec:
 
 # Learn to write a NumPy tutorial
 
-![image](https://documentation.divio.com/_images/overview.png)
-<p style="text-align:right;font-style:italic;">Image credit: Daniele Procida's <a href="https://documentation.divio.com/">The documentation system</a></p>
+![The Diátaxis framework for documentation dividing tutorials, how-to guides, explanation, and reference](https://diataxis.fr/_images/diataxis.png)
+<p style="text-align:right;font-style:italic;">Image credit: Daniele Procida's <a href="https://diataxis.fr/">Diátaxis framework</a>, licensed under <a href="https://creativecommons.org/licenses/by-sa/4.0/">CC-BY-SA 4.0</a>.</p>
 
 +++