[docs] Miscellaneous docs clean up

[colleenmcginnis]

Miscellaneous docs clean up including:

• Removing unused substitutions
• Moving images per Add table of contents scope management and refactor diagnostics docs-builder#774
• Clean up redirecting links
• Clean up ascidocc-style links

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[colleenmcginnis]

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