docs(manual): some markdown fixes

[Tachi107]

• Add a blank line before block elements (lists, code blocks, etc)
• Remove trailing whitespace
• Use "plaintext" language for code blocks when appropriate
• Fix duplicated section3_4 anchor
• Fix <` to `< in section 4.10
• Always referece images relative to the current directory
• Use markdown-style link instead of HTML href in section 7.13
• Remove extra space before closing emphasis in section 4.18

[christophe-lunarg]

Thanks for contributing !