Document: hashicorp_vault as backend when creating secret

[WanYixian]

Description

See preview of manage secrets and create secret command.

Related code PR

risingwavelabs/risingwave#22627

Related doc issue

Fix #663

Checklist

• I have run the documentation build locally to verify the updates are applied correctly.
• For new pages, I have updated mint.json to include the page in the table of contents.
• All links and references have been checked and are not broken.

[Copilot]

Pull Request Overview

This PR updates the documentation to reflect the addition of HashiCorp Vault as a backend option for creating secrets in RisingWave. The changes expand the secret management documentation to include comprehensive syntax, parameters, and examples for both the existing meta backend and the new hashicorp_vault backend.

• Added complete documentation for HashiCorp Vault backend configuration including authentication methods (token and AppRole)
• Updated syntax examples and parameter tables to cover both backend options
• Reorganized the documentation structure with clearer sections for each backend type

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
sql/commands/sql-create-secret.mdx	Added comprehensive HashiCorp Vault backend documentation with syntax, parameters, and examples
operate/manage-secrets.mdx	Updated to reference both backend options and removed outdated limitation note

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[tabVersion]

rest LGTM