Accessibility doc

[chriddyp]

• Define accessibility & link to some good 3rd party resources
• Document our aria prop support
• Document any other html & best practices like tabIndex, alt tags, semantic html tags
• Mention colorblind-friendly colorscales
• Outline known accessibility issues and provide links to the github issues. Known issues include dcc.Tabs, and DataTable. There might be more reported in the community forum
• For each known issue, describe an alternative (e.g. if dcc.Tabs isn't accessible, use links instead. If DataTable isn't accessible, use html.Table instead)
• Link to a GitHub issue for folks to report more issues