2.x javascript extender docs sometimes use return

The examples start with `export default []`, then later switch to `return []` which doesn't seem valid

https://github.com/flarum/docs/blob/1038a82ad3841ffc392d0a25c7c042267444eb0a/docs/extend/admin.md?plain=1#L17

https://github.com/flarum/docs/blob/1038a82ad3841ffc392d0a25c7c042267444eb0a/docs/extend/admin.md?plain=1#L44

https://github.com/flarum/docs/blob/1038a82ad3841ffc392d0a25c7c042267444eb0a/docs/extend/admin.md?plain=1#L64

https://github.com/flarum/docs/blob/1038a82ad3841ffc392d0a25c7c042267444eb0a/docs/extend/admin.md?plain=1#L87

https://github.com/flarum/docs/blob/1038a82ad3841ffc392d0a25c7c042267444eb0a/docs/extend/admin.md?plain=1#L107

etc...

Open docs/extend/admin.md and review the linked examples around lines 17, 44, 64, 87, and 107, checking how each extender example declares its array. Make the examples use one valid, consistent JavaScript module syntax throughout, then verify that no affected examples still use the invalid form.

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