[docs] Fix various syntax and rendering errors

[colleenmcginnis]

Fixes various syntax and rendering errors that might include:

• Fixing broken images
• Hardcoding book-level substitution values
• Fixing incorrectly closed blocks (admonitions, tab sets, code blocks, dropdowns etc.)
• Fixing poorly migrated complex tables
• Fixing poorly migrated lists
• Fixing poorly migrated tab sets
• Removing inline text formatting from directive titles where they won't be rendered (for example, inline code formatting in dropdown titles)
• Specifying if a version is trying to communicate if a feature was added, deprecated, or coming (for example, during migration deprecated:[8.15.0] became [8.15.0], which doesn't give any information about what happened in 8.15.0)
• Fixing nested dropdowns / definition lists
• Fixing poorly migrated footnotes
• Updating references to prerelease 9.0.0 versions