docs: bring back user docs in main repo (backport #22201)

[mergify]

Description

Brings back user docs on this repo (currently only https://github.com/cosmos/cosmos-sdk-docs/tree/main/docs/user)
The other repo should only be for building; otherwise it makes the scrips and the flows really weird.
Additionally, when we'll do v2, it will way easier to keep track of this.

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Author Checklist

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I have...

• included the correct type prefix in the PR title, you can find examples of the prefixes below:

• confirmed ! in the type prefix if API or client breaking change

• targeted the correct branch (see PR Targeting)

• provided a link to the relevant issue or specification

• reviewed "Files changed" and left comments if necessary

• included the necessary unit and integration tests

• added a changelog entry to CHANGELOG.md

• updated the relevant documentation or specification, including comments for documenting Go code

• confirmed all CI checks have passed

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I have...

• confirmed the correct type prefix in the PR title
• confirmed all author checklist items have been addressed
• reviewed state machine logic, API design and naming, documentation is accurate, tests and test coverage

Summary by CodeRabbit

Release Notes

• New Features

  • Added comprehensive documentation for running and interacting with blockchain nodes, including guides on keyring configuration, transaction management, and multisignature transactions.
  • Introduced a user guide for developers utilizing the Cosmos SDK, outlining essential setup and operational processes.

• Chores

  • Updated scripts to manage documentation files, including the addition and removal of specific markdown files.

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This is an automatic backport of pull request #22201 done by [Mergify](https://mergify.com).