Add details about why `extrapages` is useful

[barnjamin]

https://developer.algorand.org/docs/get-details/transactions/transactions/?from_query=apep#application-call-transaction
https://developer.algorand.org/docs/get-details/transactions/
https://developer.algorand.org/docs/get-details/dapps/smart-contracts/apps/?from_query=state%20budget#the-lifecycle-of-a-smart-contract

We should mention that extra pages is useful to set if the dev expects they might push an update that'd push the program requirements past a single page since the setting is immutable

[FrankSzendzielarz]

Also to emphasise that the max program size is immutable, which should be a consideration at the outset if the developer plans to extend the contract/app