docs: move around upgrade docs

[julienrbrt]

Description

Working on #15120 made me look at upgrade docs.
Which explains the workaround described in #15120 fairly extensively, but that is another issue 😅

• https://docs.cosmos.network/main/core/upgrade#overwriting-genesis-functions
• https://github.com/cosmos/cosmos-sdk/blob/main/types/module/module.go#L619-L623

However, I have noticed a quick improvement could be made to the general SDK upgrade docs, by simply moving the well written extensive x/upgrade/doc.go to display in our documentation.

ref: #11504

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[tac0turtle]

thanks for moving this