Add more examples for "Adding Your Own Startup Scripts" on the Spin Docs

[luchtech]

👉 Describe the problem

I was trying to find a way to run an Artisan command on every deployment. I've been staring at the Spin docs but I can't see it there. Turns out that it is documented on another docs site: https://serversideup.net/open-source/docker-php/docs/customizing-the-image/adding-your-own-start-up-scripts

👥 Problem evidence & reach

I had to ask the Discord "spin" channel and @jaydrogers pointed me to that link above.

🥰 Describe the "impact" on users?

I'm pretty sure that a lot of Laravel devs need to run some Artisan commands on every deployment so making this document more visible on the Spin docs will surely help.

🏆 How to solve this problem

Maybe provide examples plus reiterate the purpose of AUTORUN_ENABLED to make sure they don't run the same commands again.