Docs: github-mode: Hint at different env var names

[tordans]

The NEXT_PUBLIC_KEYSTATIC_GITHUB_APP_SLUG is only the right name for Next.JS integrations.
This PR adds a hint for Astro users to look out for a different variable.

---

Side note: I wonder if this condition should be extended to handle both variables as well 618ba4a#diff-8b4b55d3c75b76109c771dcd8a01e1b1339c07b753bb409b93e71df2f9d4d2cfR91

[changeset-bot]

⚠️ No Changeset found

Latest commit: 097f21d

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[tordans]

FYI: This change happened automatically when editing via Github UI

[emmatown]

Side note: I wonder if this condition should be extended to handle both variables as well 618ba4a#diff-8b4b55d3c75b76109c771dcd8a01e1b1339c07b753bb409b93e71df2f9d4d2cfR91

No, this was a specific thing to give users a hint at what might be going wrong because that commit required updates to both @keystatic/core and @keystatic/next but not other framework packages