Checklist enhancements

See: https://github.com/a11yproject/a11yproject.com/pull/779#issuecomment-491527673

• Consider adding a dos and don'ts section to each checklist item.
• Consider adding a "further reading" section to each checklist item.
• Consider referencing each checklist item in a longer, more detailed post explaining why it's important.
• Provide a toggle to expand every item in checklist section/all checklist items on the page.