Apply accessibility guidelines to splink documentation

[mamonu]

• [ ] Change the current default font to an accessible font that follows Web Content Accessibility Guidelines (WCAG) 2.1.There is functionality to easily do this with mkdocs

• [ ] The visual presentation of text needs to have a contrast ratio of at least 4.5:1 on all colour palettes (light/dark)

[mamonu]

additionally there is a way to automatically test for accessibility on websites via the node.js package Pa11y. Mostly everything passes on our documentation. Running it on one of the notebook tutorials however gives

 • Error: Img element missing an alt attribute. Use the alt attribute to specify a short text alternative.
   ├── WCAG2AA.Principle1.Guideline1_1.1_1_1.H37
   ├── html > body > div:nth-child(8) > main > div > div:nth-child(3) > article > div:nth-child(15) > div:nth-child(2) > div > div > div > img
   └── <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAA6QAAAh6CAYAAACbjNB4AAAAAXNSR0IArs4c6QAAIABJREFUeF7snQnYXsPZxyeI2CO2oKEpgqKUNqittdSui4ooTS1FtbS0tTSf1q5pVa0pRZVPlYaitdS+tBKRamlV1RKfJZbEGklskch3/Q4T533ybOfcc94zz3n+c125SJ4zc+75zT33zH9mzjl95syZM8...

So yeah. need to add alt-text to the images we have on the notebooks