Adjust ad insertion script to give space around interactives

[frederickobrien]

Ads in interactive articles sometimes disrupt the narrative flow by breaking up key information blocks. E.g.:

Have spoken with Commercial and no initial objections to updating the ad placeholder script used on apps:

https://github.com/guardian/dotcom-rendering/blob/main/dotcom-rendering/src/model/enhance-ad-placeholders.ts#L154