Refresh Astro example

[sylwiavargas]

Summary

This PR refreshes the Astro example:

• added a button so users can more easily interact with Inngest
• added instructions on the landing page
• added comments in the code
• expanded instructions in the readme
• changed the npm run dev command so Inngest automatically connects to the Astro app (Astro does not expose the network by default).

[changeset-bot[bot]]

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Latest commit: 0750e1b78f0e3d4590a6fcab27ae851e76dbabdc

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