docs: add example command for GitHub token usage in sp1up

[VolodymyrBg]

Motivation

The installation documentation mentions the ability to use a GitHub token with sp1up to avoid rate-limiting issues, but it doesn't provide a concrete example of how to use this feature. This lack of example might confuse users who are trying to resolve rate-limiting issues.

Solution

Added a clear example command showing how to use the --token flag with sp1up, making it immediately obvious how users should format their command when using a GitHub token.

The example uses a placeholder token format that follows GitHub's standard pattern (ghp_YOUR_GITHUB_TOKEN_HERE), making it clear where users should insert their actual token.

PR Checklist

• Added Tests
  • N/A (Documentation change only)
• Added Documentation
  • Added example command in installation guide
• Breaking changes
  • No breaking changes