Clean up markdown in omnibus docs (part 1)

What does this MR do?

Clean up various markdown issues in omnibus docs, (not including /settings, see !3454 (merged)), including whitespace, spacing, headers and code blocks.

This is in preparation for running an expanded list of rules in the markdown lint:

• MD005 Inconsistent indentation for list items at the same level
• MD007 Unordered list indentation
• MD009 Trailing spaces
• MD010 Hard tabs
• MD012 Multiple consecutive blank lines
• MD022 Headers should be surrounded by blank lines
• MD025 Multiple top level headers in the same document
• MD026 Trailing punctuation in header
• MD027 Multiple spaces after blockquote symbol
• MD028 Blank line inside blockquote
• MD030 Spaces after list markers
• MD031 Fenced code blocks should be surrounded by blank lines
• MD038 Spaces inside code span elements
• MD046 Code block style

Related to #4537 (closed)