[NewMayur]

Summary

This PR significantly expands and modernizes the Key Store user guide (docs/user-guide/key-store.md), documenting several critical but previously undocumented features present in Semaphore UI:

1. Built-in SSH Key Generator:
   • Documents the one-click Generate SSH Key checkbox within the credential modal.
   • Clarifies that checking this box disables manual private key input and instructs the backend to generate a secure ED25519/RSA keypair automatically.
2. Public Key Extraction Workflows:
   • Details the Generated SSH Public Key popup dialog that appears immediately after generation with a 1-click clipboard copy button.
   • Documents the expandable Public key drawer (Show/Hide preview) that appears when subsequently editing existing generated keys (hasGeneratedPublicKey), so users never have to guess where their public keys went.
3. Secret Masking & Override Protection:
   • Explains that password, passphrase, and private key fields are masked by default with reveal controls (mdi-eye).
   • Documents the mandatory Override checkbox (item.override_secret) that must be checked when editing an existing credential to prevent accidental secret overwrites.
4. Login with Password Dual-Use:
   • Explicitly explains how leaving the Username field empty transforms the credential into a standalone secret string (for Ansible Vault passwords, API Bearer tokens, or webhook secrets).
5. Read-Only Secret Storage Notice:
   • Documents the Read-only secret storage chosen. notice rendered when selecting the Env or File source tabs.
6. Visual Assets:
   • Adds 4 clean WebP screenshots captured directly from Semaphore UI:
     • key-store-generate-ssh.webp
     • key-store-generated-public-key.webp
     • key-store-public-key-drawer.webp
     • key-store-env-readonly-storage.webp