Replace all reference to custom folder with ext in the FE v4 documentation

[nlunets]

We've noticed that all samples in the V4 world were using custom as folder name while the guidelines should be ext
This PR fixes that

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

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[KvM2]

Hello , thanks for your contribution! We really appreciate your contribution and will look into this .

[JannaLisa]

Hi @nlunets ,
Sorry for not having done this yet, and also for not responding for such a long time :-(
Will discuss with the team whether someone can take this up asap.

[mishuagrawal]

Hello,

Please accept our sincere apologies for the delay in updating this request and the documentation. We have now fixed all the mentioned files. The changes should start reflecting post the next release.

I will be closing this request now. In case you have any further questions or concerns, please feel free to write back.

Thanks and regards,
Mishu Agrawal