Enhanced Astro codespace support (#6288)

* Enhanced Astro codespace support

Enhances the base devcontainer and Dockerfile to provide a smoother
development experience out of the box for contributors.

Adds a devcontainer per example so that we can add buttons on astro.new
that will launch users straight into the appropriate example app.

* Cleanup devcontainers and pin base image

* Update .devcontainer/example-welcome-message.txt

Co-authored-by: Samruddhi Khandale <skhandale@microsoft.com>

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Co-authored-by: Samruddhi Khandale <skhandale@microsoft.com>