🔧 chore(github): update github app setup instructions for sentry.conf.py (#15047)

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The instructions for setting up github weren't clear if you are using
`sentry.conf.py`, so i added a quick note. I also put a reminder for a
common mistake people make when copying their secret into their config

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Author: iamrajjoshi

develop-docs/integrations/github.mdx

@@ -95,3 +95,35 @@ github-app.private-key: |
   privatekeyprivatekeyprivatekeyprivatekey
   -----END RSA PRIVATE KEY-----
 ```
+
+You can also use a `sentry.conf.py` file to store your configuration as follows:
+
+```python
+from sentry.conf.server import SENTRY_OPTIONS
+
+# App ID
+SENTRY_OPTIONS["github-app.id"] = GITHUB_APP_ID
+# App Name
+SENTRY_OPTIONS["github-app.name"] = GITHUB_APP_NAME
+# Client ID
+SENTRY_OPTIONS["github-app.client-id"] = GITHUB_CLIENT_ID
+# Client Secret
+SENTRY_OPTIONS["github-app.client-secret"] = GITHUB_CLIENT_SECRET
+# Webhook Secret
+SENTRY_OPTIONS["github-app.webhook-secret"] = GITHUB_WEBHOOK_SECRET
+
+SENTRY_OPTIONS[
+    "github-app.private-key"
+] = """
+-----BEGIN RSA PRIVATE KEY-----
+privatekeyprivatekeyprivatekeyprivatekey
+privatekeyprivatekeyprivatekeyprivatekey
+privatekeyprivatekeyprivatekeyprivatekey
+privatekeyprivatekeyprivatekeyprivatekey
+privatekeyprivatekeyprivatekeyprivatekey
+-----END RSA PRIVATE KEY-----
+"""
+
+```
+
+Take note that your private key should be a multiline string without any whitespace before the start of a new line of the key.