Add oauth details to docs

Author: tcompa

docs/oauth2.md

@@ -13,3 +13,27 @@ PUBLIC_OAUTH_CLIENT_NAME=github
 ```
 
 This variable has to match the client name defined in fractal-server configuration.
+
+
+## Full details for local example
+
+1. Register an OAuth2 app on GitHub with _Homepage URL_ http://127.0.0.1:5173 and _Authorization callback URL_ http://127.0.0.1:5173/auth/login/oauth2/.
+2. Set up the following env variables for fractal-server:
+```
+OAUTH_GITHUB_CLIENT_ID=...
+OAUTH_GITHUB_CLIENT_SECRET=...
+OAUTH_GITHUB_REDIRECT_URL=http://127.0.0.1:5173/auth/login/oauth2/
+# [all other variables]
+```
+3. Set up the following env variables for fractal-web:
+```
+FRACTAL_SERVER_HOST=http://127.0.0.1:8000
+AUTH_COOKIE_DOMAIN=127.0.0.1
+PUBLIC_OAUTH_CLIENT_NAME=github
+# [all other variables]
+```
+
+> A typical gotcha: if there is a mismatch between the cookie domain and the
+> URL you are using (e.g. one points to localhost and the other one to
+> 127.0.0.1), then the cookie won't be set and this will fail silently,
+> therefore likely triggering other unexpected behaviors.