Docs: github-mode: Simplify redirect_uri instructions

This is a follow up to Thinkmill#1358 which makes the steps to get to the app easier.

docs/src/content/pages/github-mode.mdoc

@@ -124,16 +124,17 @@ When you authorize on a server and get the following error from GitHub, you need
 
 To add a redirect URL:
 
-1. Go to the [list of "Installed Github Apps"](https://docs.github.com/en/apps/using-github-apps/reviewing-and-modifying-installed-github-apps#navigating-to-the-github-app-you-want-to-review-or-modify)
-   - For Users: [`https://github.com/settings/installations`](https://github.com/settings/installations)
-   - For Organisations: `https://github.com/organizations/<org name>/settings/installations`
-2. Select the app > Choose "App settings"
-   - For Users: `https://github.com/settings/apps/<app slug>`
-   - For Organisations: `https://github.com/organizations/<org name>/settings/apps/<app slug>`
+1. Open your app page using values of the `PUBLIC_KEYSTATIC_GITHUB_APP_SLUG` environment variable
+   `https://github.com/apps/<app slug>`
+2. Select "App settings"
 3. Use "Add Callback URL" > Add the additional URL > Save
 
 Now try reloading the authentication page.
 
+If you don't have the app slug, use the [list of "Installed Github Apps"](https://docs.github.com/en/apps/using-github-apps/reviewing-and-modifying-installed-github-apps#navigating-to-the-github-app-you-want-to-review-or-modify) to find your app.
+- For Users: [`https://github.com/settings/installations`](https://github.com/settings/installations)
+- For Organisations: `https://github.com/organizations/<org name>/settings/installations`
+
 ---
 
 ## Deploying Keystatic