Add Gemini CLI support to documentation

[dandye]

Summary

• Added Gemini CLI as a supported MCP client across all documentation
• Updated configuration examples to include Gemini CLI alongside Claude Desktop
• Added setup instructions specific to Gemini CLI

Changes

• Updated main README.md with Gemini CLI configuration sections
• Modified docs/usage_guide.md to include Gemini CLI in prerequisites and setup
• Updated docs/index.md to mention Gemini as a compatible AI assistant
• Added Gemini CLI configuration examples to all server README files
• Updated all server documentation files in docs/servers/ with Gemini CLI examples
• Added note in run-with-google-adk/README.md directing users to main README for other clients

Test plan

• Review all documentation changes for accuracy
• Verify configuration examples are consistent across all files
• Check that Gemini CLI setup instructions are clear and complete