[Sftp] Add module help page

[VeryEarly]

Description

Mandatory Checklist

• Please choose the target release of Azure PowerShell. (⚠️Target release is a different concept from API readiness. Please click below links for details.)

  • General release
  • Public preview
  • Private preview
  • Engineering build
  • No need for a release

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD update ChangeLog.md file(s) appropriately
  • Update src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md.
    • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT adjust version of module manually in pull request

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[Copilot]

Pull Request Overview

This PR adds a module help page for the Az.Sftp PowerShell module, providing documentation and cmdlet descriptions for SFTP functionality in Azure Storage.

Key Changes

• Creates the main module help page with module description and cmdlet summaries
• Documents two cmdlets: Connect-AzSftp for establishing SFTP connections and New-AzSftpCertificate for generating SSH certificates
• Provides reference link to Microsoft documentation for Azure Storage SFTP support