Adjust Mermaid block styling and block content

[chalin]

• Context: Use <pre> tag for mermaid code blocks #1451
• As a <div> the Mermaid code block has no extra styling.
  Now as a <pre> it does -- extra padding all around, in particular -- since it applies to all td pre elements.
• We should also avoid introducing extra whitespace the <pre> element: use {{- .Inner -}}