[rust] Simplify SM readme

[bonigarcia]

Description

This PR updates the Selenium Manager README page.

Motivation and Context

Now we have a comprehensive documentation page for Selenium Manager, so the current SM README page is outdated. It is better to simply link to the documentation page, to avoid maintaining two doc sources. Besides, the published SM roadmap (M1, M2, etc.) has no sense anymore.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.