Docs accessibility

[ygannett]

Add general overview of web accessibility. Currently, the content is in a possible reading order, which I think it makes sense as a flow.

• Add all possible components.
• Some links don't work since their linked URLs haven't been added yet.
• "Disability types" is also set up, but not included in this file.

To do:

• Add general high level implementation of web accessibility.
• Set up a structure for the accessibility section, and spread the content in multiple files.
• Put together all source links in one place, not per section.
• Set up the requirements/policies Mayflower meets. (Mass.gov specific requirements)

The original content in the file is shown below the newly added one.

[clairesunstudio]

Overall this is a good start and thorough research on the providing the context for accessibility! Content can be repetitive and need be more succinct (maybe pass it to the content team to work on). I would recommend making an accessibility folder and move this under it as an intro/background (could also do this in the gitbook editor). We will probably also need an overview for TL;DR