Fix typos in guidelines/index.html

[kfranqueiro]

This fixes several types of mistakes I noticed in the WCAG 3 Editor's Draft:

• Resolves invalid nesting of lists inside of paragraphs by ending the paragraph before the list begins
• Corrects heading end tags (e.g. closed too soon, or with start and end tags that didn't match)
• Updates instances of provision torequirement in data attributes or classes
• Adds class="requirement-text" to an unordered list that continues on from an initial paragraph in Complete tasks (we may want to use a wrapper element instead)
• Fixes text in methods link for Equivalent text alternative
• Updates capitalization in "How to meet" links to match guideline names
• Removes a "(Needs additional research)" parenthetical which is redundant of the adjacent Editor's Note
• Removes redundant data-lt values (plurals and entries that exactly match the dfn element's text content)
• Cleans up whitespace

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