[5.2] Use classes instead of IDs

[timacdonald]

Collection and helper function heading tags are currently broken for several older versions of the docs. This PR targets 5.2.

On server render, the heading tags all contain the same literal `id="collection-method" attribute; IDs are meant to be unique.

<h4 id="collection-method"><code>random()</code></h4>

When the JavaScript kicks in, the ID attribute is updated its corresponding <a name="..."> link, however the intended styling is then broken because it targets the id attribute, e.g., #collection-method { ... }.

• Matches the later documentation versions
• Gets the intended styling applied
• Has no flash of styled / unstyled content