Added some details on what an entitlment.plist file should contain and why wrong entitlements can make a build crash.

Also updated links to the default entitlement.plist as the previous one was giving a 404 (it's been moved adn renamed). Not sure if I linked the right files though. Maybe adding a link to the whole `entitlements` [folder](https://github.com/electron/osx-sign/tree/main/entitlements) instead  would be more suitable, as it contains self-explanatory filenames for the different usecases ?

Thanks to @jeremyspiegel @mmaietta and @idanwork for helping pinpoint the issue.

Closes #7050

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