chore: Remove Gitpod/Ona info

[erbierc]

Description (required)

In Starlight, during a PR a discussion emerged about Gitpod and how it has become Ona, a pay-to-use AI-development platform. Gitpod was removed from the CONTRIBUTING.md file there and so I would like to continue with that for Astro docs as well.

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Note: It would also be a good idea to remove any gitpod related files, such as .gitpod.yml.

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[ArmandPhilippot]

Thank you for reporting the changes here as well! I think you can safely add the gitpod.yml file in this PR!

Now, I'm not sure if we want to remove or replace the Ona button. I'll ask to the rest of the team before we merge this one.

While reviewing the CONTRIBUTING.md file to check if we had missed another occurrence, I noticed another (unrelated) sentence that should be updated in "Pages generated from outside sources". I don't think this requires another PR and you could add this into this one if you want:

-Currently, these files are configuration-reference.md and all those under the integrations-guide/ directory as well as our error messages.
+Currently, these files are configuration-reference.md and all our error messages.

The integrations-guide directory is no longer auto-generated.

[erbierc]

Done and done, absolutely take your time with the discussions!

[ArmandPhilippot]

Thank you for the quick updates. We agree to simply remove the button, so everything LGTM! 🚀