Evaluate mentions of Gemini CLI and Code Assist for updates to Antigravity

[parlough]

For individual developers, Gemini CLI and Gemini Code Assist will be superseded by Antigravity and Antigravity CLI. We should add documentation for Antigravity CLI and consider replacing or removing the Gemini CLI and Gemini Code Assist docs. It seems Gemini CLI and Gemini Code Assist will continue on in some form for enterprise customers, so keeping some mentions might make sense.

References:

• An important update: Transitioning Gemini CLI to Antigravity CLI
• Google Antigravity CLI
• Migrating from Gemini CLI
• Google Antigravity in Gemini Enterprise