Document Antigravity 2.0 and Antigravity IDE as separate tools

[parlough]

Antigravity 2.0 is focused on the agentic experience while Antigravity IDE remains as an editor with the agent manager. Eventually it seems the agent manager will be removed and will transition to a focused editor.

As part of this, we should ensure the existing instructions are still up to date for Antigravity IDE as there might have been changes.

References:

• Introducing Antigravity 2.0
• Antigravity 2.0 docs
• Antigravity IDE docs