docs(updater): Update authentication documentation for ssh-key and api-token inputs

[vaind]

Summary

Updates the updater README to accurately document the ssh-key and api-token authentication inputs that were added in v3.1.0.

• Fix api-token input documentation: change from required: true to required: false
• Add comprehensive documentation for ssh-key input parameter
• Add usage examples showing ssh-key alone and combined with api-token
• Add Authentication section with three clear options and guidance on when to use each
• Clarify that CI runs on PRs when using SSH keys for git operations

Changes

Updated Inputs Section

• api-token now correctly shows required: false (was incorrectly required: true)
• Added ssh-key input documentation explaining it can be used alone or with api-token

New Usage Examples

• Example using SSH key authentication only
• Example using both SSH key + API token together

New Authentication Guide

• Option 1: API Token Only (default) - for standard GitHub token auth
• Option 2: SSH Key Only - for deploy keys (CI will run on PRs)
• Option 3: SSH Key + API Token - for full control over both git and API auth

Test plan

• Verify documentation accurately reflects the action.yml inputs
• Verify examples show all authentication methods
• Verify authentication guide provides clear guidance

🤖 Generated with Claude Code