[8.x] Fix duplicated ids when rendering, missing style definitions

[driade]

This pull requests tries to fix some errors found when rendering the docs with Parsedown.

• First, using #collection-method and Parsedown (https://github.com/laravel/laravel.com-next/blob/master/app/Parsedown.php) at laravel.com-next, renders a duplicated id

<h4 id="method-all" id="collection-method" class="first-collection-method"><code>all()</code></h4>

which causes and invalid html. It's not a very big issue unless you're trying to make an epub out it (please see the screenshot)

• Second, trying to follow the idea behind collections.md, where the methods have a class to make the "code" tag render with 14px, and having the first element also the class ".first-collection-method", I've checked a few files where the css style was missing or the class was not correctly added to the code.

If accepted this pull requests should be backported.

Thanks.