Mention Project IDX editor and document its Flutter capabilities

[parlough]

Some of this may just be notes alongside or additions to other VS Code mentions, but in the future it may benefit from some of its own documentation, especially some things like previews on the mobile simulators, or deployments to Firebase hosting.

Edit: Perhaps a dedicated flow in "Get Started" would make sense too.

Announcement: https://developers.googleblog.com/2023/08/introducing-project-idx-experiment-to-improve-full-stack-multiplatform-app-development.html Website: https://idx.dev/

[atsansone]

@anderdobo : Do you have a timeline envisioning when we should add this?

[anderdobo]

@atsansone I don't have a timeline on this yet. We can assess need at the time IDX evolves beyond public preview.