docs(keyring): add keyctl docs (backport #23563)

[mergify]

Add user documentation about the relatively new keyctl backend introduced in Cosmos SDK

Supersedes cosmos/cosmos-sdk-docs#270

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Summary by CodeRabbit

• New Features

  • Added documentation for the keyctl backend, a Linux-specific key storage option that leverages the kernel's security key management system.

• Documentation

  • Updated keyring documentation with improved descriptions and clarity about available backend options.

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This is an automatic backport of pull request #23563 done by [Mergify](https://mergify.com).