Fix markdown in runner docs (part 1)

What does this MR do?

Clean up various markdown issues in the runner docs, 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 #4523 (closed)