Add custom logo configuration (#209)

docs/configuration/cheat-sheet.md

@@ -30,3 +30,5 @@
 | oidc.client-key       | OG_OIDC_CLIENT_KEY       | none                  | The client key for the OpenID application.                                                                                                                                                                                       |
 | oidc.secret           | OG_OIDC_SECRET           | none                  | The secret for the OpenID application.                                                                                                                                                                                           |
 | oidc.discovery-url    | OG_OIDC_DISCOVERY_URL    | none                  | Discovery endpoint of the OpenID provider.                                                                                                                                                                                       |
+| custom.logo           | OG_CUSTOM_LOGO           | none                  | Path to an image, relative to $opengist-home/custom                                                                                                                                                                              |
+| custom.favicon        | OG_CUSTOM_FAVICON        | none                  | Path to an image, relative to $opengist-home/custom                                                                                                                                                                              |