Add guidance for mark role

[mcking65]

Updates two practice pages to add information about mark role:

1. On page "Structural Roles", adds mark role to table in section "All Structural Roles and Their HTML Equivalents".
2. On page "Providing Accessible Names and Descriptions", adds mark role to table in section"Accessible Name Guidance by Role".

Fixes #1821.
Replaces PR #1826.

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• Editorial review by reviewer_id
• N/A: Functional review by reviewer_id
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• N/A: Test review by reviewer_id

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[jnurthen]

LGTM