Atif Ali e6e2d9789e docs: mention making the GitHub App public and APP_INSTALL_URL (#25188) ...

## Summary

The GitHub App walkthrough in `docs/admin/external-auth/index.md` stops
after \"install the app for your organization,\" which is enough for the
admin who created the app but not for anyone else. Every other Coder
user hitting **Link GitHub** lands on a GitHub 404 (`This is not the web
page you are looking for`) because:

1. New GitHub Apps default to **\"Only on this account\"** / not public.
GitHub returns 404 from the OAuth-authorize URL for any user other than
the owner.
2. `CODER_EXTERNAL_AUTH_0_APP_INSTALL_URL` — the env var that makes
Coder render an \"Install GitHub App\" link in the UI — is undocumented
today.

This PR adds one extra step at the end of the GitHub App configuration
walkthrough covering both.

## Test plan

- [x] \`make fmt/markdown\` clean
- [x] Doc reviewer eyes

2026-05-12 15:02:00 +00:00

..

index.md

docs: mention making the GitHub App public and APP_INSTALL_URL (#25188)

2026-05-12 15:02:00 +00:00