Docs Infra: Remove borders from API page

[timdeschryver]

Information

The API docs have too many borders drawn around some blocks.
A cause can be because the current DOM structure renders the following DOM structure, with a nested code/pre block:

<pre>
  <code>
    <pre>
      <code>
      </code>
    </pre>
  </code>
</pre>

Documentation page

https://ngrx.io/api/effects/provideEffects

I would be willing to submit a PR to fix this issue

• Yes
• No

[santoshyadavdev]

Checking this