## 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
**Breaking Change:** Existing oauth apps might now use PKCE. If an
unknown IdP type was being used, and it does not support PKCE, it will
break.
To fix, set the PKCE methods on the external auth to `none`
```
export CODER_EXTERNAL_AUTH_1_PKCE_METHODS=none
```
Solves #15575
Adds OAuth access token revocation when unlinking external auth
provider. Due to revocation not being consistently implemented by
providers this is only best effort attempt. Unsuccessful revocation
won't influence link removal.
Atif Ali