Add new section describing interaction between and precedence of IDL attributes/HTML attributes

[rahimabdi]

In a recent ARIA WG group meeting, it was proposed to add an html-aam section describing IDL overrides for HTML attributes and what accessibility values are actually exposed. Examples:

• indeterminate IDL attribute correlates with aria-checked: https://w3c.github.io/html-aam/#att-indeterminate
• validity IDL attribute correlates with aria-invalid: [html-aam PR 395] add validity IDL aria#2223
• placeholder IDL attribute (currently missing in html-aam but should be added?): https://w3c.github.io/html-aam/#att-placeholder

Additional notes:

• Reflection should not impact values exposed by accessibility APIs; for example, el.ariaChecked would reflect the aria-checked content attribute, and would be unimpacted by the indeterminate IDL attribute
• Where a definition is needed for an IDL attribute that correlates to an ARIA attribute, it should ideally use the "underlying concept" rather than reference the IDL attribute. For example, the validity IDL attribute has this underlying definition: https://html.spec.whatwg.org/#selector-valid