Documentation for channel upgradability

[crodriguezvega]

Write documentation for channel upgradability (maybe in a subsection of Using IBC-Go).
Each task may be tacked in a separate PR.

Tasks

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1. Document reasoning and use cases for channel upgradability #5267
   channel-upgradability docs +2
   
2. Document how upgrade timeout works #5268
   channel-upgradability docs +2
   
3. Document packet flushing #5269
   channel-upgradability docs +2
   
4. Document usage of authority-gated MsgChannelUpgradeInit #5270
   channel-upgradability docs +2
   
5. Document the differences of submitting MsgChannelUpgradeCancel signed by a relayer or an authority. #5271
   channel-upgradability docs +2
   
6. Document what application modules need to do to implement the upgrade callbacks. #5272
   channel-upgradability docs +2
   
7. Tutorial for upgrading an existing transfer channel to wire up fee middleware. #5273
   channel-upgradability docs +2
   
8. Document ibc app expectations and recommendations for channel upgrades #4277
   04-channel channel-upgradability docs +3
   
9. Document usage of message for pruning acknowledgements #5501
   channel-upgradability docs +2