docs: create v2 upgrading guide skeleton (backport #22696)

[mergify]

Description

ref: #22531

I am upgrading Ignite CLI to support v2 scaffolding and modules (ignite/cli#4432 & more), so I best can just document everything needed here as well :)

See #22696 (comment)

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Summary by CodeRabbit

• New Features
  • Introduced a comprehensive upgrade guide for transitioning to Cosmos SDK v2.0.0.
• Documentation
  • Added UPGRADING_v2.md detailing migration processes and key changes for users upgrading from earlier versions.

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This is an automatic backport of pull request #22696 done by [Mergify](https://mergify.com).

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