Add upgradeability pattern doc for contracts

[joelpeace48-cell]

Document how we could make the rewards or campaign contract upgradeable (e.g. deployer pattern, migration steps) and add a short "Future: upgradeability" section in docs.

[D2758695161]

Crab Bounty Claim

Submitted PR #135 for this issue. Payment address: 0xaae0101ac77a2e4e0ea826eb4d309374f029b0a6

[JemimahEkong]

@JemimahEkong has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

Hi Maintainer, kindly assign this issue to me. I have the skill and experience to confidently deliver.

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[anupam7984]

@anupam7984 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I would like to take this issue and can solve in 24hrs. I am full stack web2+web3 deveoper.

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[Godbrand0]

@Godbrand0 has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

hello, i would like to work on this issue

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[drips-wave]

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