docs: correct outdated fallback keyring path documentation

[CygnusMaximillian]

📝 Description

This PR updates the File-based Keyring (Fallback) documentation in the Harbor CLI Encryption Guide to reflect standard XDG Base Directory Specification alignment.

Previously, the documentation erroneously stated that the default fallback keyring file path defaults to a top-level home dotfile directory (~/.harbor/keyring). Local isolation testing on modern distributions verifies that the CLI binary strictly respects system-wide XDG variables (~/.config/harbor-cli/config.yaml and ~/.local/share/harbor-cli/), never generating a legacy ~/.harbor/ folder in the user's root home path.

🛠️ Changes Made

• Modified: doc/cli-encryption/_index.md
• Removed out-of-date documentation references targeting ~/.harbor/keyring.
• Replaced outdated path with a generalized, clear note directing users toward the active XDG-compliant folder hierarchy (e.g., ~/.local/share/harbor-cli/keyring or ~/.config/harbor-cli/keyring).

🧪 Verification & Testing Completed

1. Environment Stripping: Executed local configuration initializations using an isolated context wrapper to block default desktop session ring hooks (env -i HOME=$HOME PATH=$PATH ./harbor-cli login...).
2. Footprint Check: Verified that directory generation successfully mapped variables to .config/harbor-cli/config.yaml and .local/share/harbor-cli/data.yaml without generating an extraneous ~/.harbor file path.
3. Markdown Validation: Confirmed documentation links and styling render without any lint errors.

🔗 Related Issues

Fixes #934