Support `apps.importable.tsx` files

[sophie-macmillan]

According to these recommendations ](https://github.com/guardian/dotcom-rendering/blob/main/dotcom-rendering/docs/contributing/where-should-my-code-live.md#L63)we should be able to name files <NAME>.apps.importable.tsx to distinguish which renderingTarget the code is for

This currently isn't working though, so we have (at least) 2 files that don't follow convention. We should fix this.

• AppsFooter.importable.tsx
• AdsPortal.importable.tsx

[alinaboghiu]

quick fix: remove this advice from the readme 👀

[mxdvl]

I think that having the name of the component containing the platform it is meant to exclusively render on is beneficial. Otherwise, it can easily be missed that a component was meant for a specific platform.