Add alt-text to images

[bsipocz]

To make the webpage more accessible. This is really just an umbrella issue as addressing it belongs to all the 3 repos that actually contains the elements of the website.

Some resources about alt-text:

• https://hackmd.io/2uOMXV_fT0ycLyo4uH67HQ
• https://docs.google.com/presentation/d/1PTYVwcu2U6wTY-ozrKEK981CEp79MkBtRJymr2LQgCY/edit#slide=id.gc6f9e470d_0_0

[bsipocz]

The three repos where the content is generated for the website. Any alt-text solutions should go into those repos.

https://github.com/astroML/astroML - rst files, as well as some API docs (though maybe there are no figures there)
https://github.com/astroML/astroML_figures - in the book_figures directory
https://github.com/astroML/astroML-notebooks - notebooks that are rendered into HTML