Accessibility page tidy up

Some sections of the [accessibility page](https://dfe-analytical-services.github.io/analysts-guide/learning-development/accessibility.html) could be made clearer or more informative

- Add a brief section at the top to explain different accessibility needs (visual, auditory, motor, cognitive) with info from this presentation (and probably a link to the presentation because it explains things nicely): [https://www.youtube.com/watch?v=hRR2VNFFLek](https://www.youtube.com/watch?v=hRR2VNFFLek). Also worth noting that some accessibility needs can be temporary.
- Add a sentence at the top of "colour accessibility tools" to explain why you'd want to check contrast, simulated colour blindness and greyscale. There's some info in the AF link at the bottom of the section but that relies on users leaving the site to understand what they're checking for and why and I don't think everyone will put that much effort in
- Making spreadsheets accessible section could do with a reorder, it mentions "this guidance" in first paragraph with no link to guidance or reference to what "this guidance" actually is until the next subheading. The heading levels are also a bit off I think, could all do with being subheadings under one "making spreadsheets accessible" heading rather than under 3 separate subheadings
- Find and add a definition of "cognitive load" in the relevant section