Add normative text on calculating aria-level, aria-posinset, and aria-setsize

[smhigley]

Closes #1194

As discussed in the issue, this PR adds normative wording on requiring user agents to calculate aria-level, aria-posinset, and aria-setsize. It also adds a section outlining how to compute each attribute, based on the current Core AAM wording that will be removed after this is merged.

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• Related Core AAM Issue/PR:
• Related AccName Issue/PR: N/A
• Any other dependent changes? N/A

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• Related APG Issue/PR: N/A
• MDN Issue/PR: N/A
• "author MUST" tests: N/A
• "user agent MUST" tests: issue created at Write tests for implicit value calculation for ARIA positioning attributes, once testing is unblocked web-platform-tests/interop-accessibility#99
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