Add "edit in playground" to stlite embeds

[arnaudmiribel]

Changes

Preview docs

• You can now hit "Edit in playground" to try out an extra with source code in Streamlit playground
• Refactored extras metadata to merge __experimental_playground__ (deprecated) and __stlite__. Merged into __playground__
• Docs now use __playground__ and also redirect folks to the playground if they want to edit the embedded stlite
• Edited one or two extras to unlock their playgrounds, hopefully - it's hard to test since stlite can't use packages from source, I basically need a version upgrade to test this out eheh

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