Adds in a section for every rule that specifies what the WCAG guidelines are (if applicable) in a new Accessibility guidelines section near the bottom of the rule documentation.

Additional changes:
- Move Resources/References underneath Accessibility guidelines section
- Fix up heading level of Resources so that the heading order progresses sequentially
- Fix up resource link text to be clearer
- Fix up some broken links or old references
- Update pronoun usage in `accessible-emoji` to use they instead of he/she
- Add some additional examples and rule clarity to `click-events-have-key-events`
- Fix up code formatting for `tabindex-no-positive`
- Add additional resources to some rules without resources
- Change Resources from ordered list to unordered list since the order really doesn't matter

Partially addresses jsx-eslint#30