doc(openid-connect): Add initial setup instructions (#7174)

* doc(openid-connect): Add initial setup instructions

See discussion for reference: https://github.com/FreshRSS/FreshRSS/discussions/5684#discussioncomment-11707635

* style(openid-connect): Change lists to dash style

* Minimize changes

* Typography

---------

Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

docs/en/admins/16_OpenID-Connect.md

@@ -10,13 +10,30 @@ Additional documentation can be found in that project.
 
 The callback URL is `https://<your-domain>/i/oidc/`.
 
+## Initial Setup Process
+
+When setting up a new FreshRSS instance with OIDC, follow these steps carefully to ensure proper administrator access:
+
+1. Configure your OIDC environment variables (see configuration section below)
+2. Start your FreshRSS instance
+3. Access the Web interface – it will immediately attempt to authenticate you via your OIDC provider
+4. After successful authentication, you’ll be directed to the setup wizard
+5. In the authentication setup step (currently *step 4*):
+   * Enter the exact username that matches your OIDC identity (e.g., `admin@idm.example.com`) as the default user
+   * The password field can contain any random value as it won’t be used with OIDC
+   * Select *HTTP Authentication Method* as the authentication method
+   * If configured correctly, you should see your current username displayed as: `HTTP (for advanced users with HTTPS) (REMOTE_USER='admin@idm.example.com')`. If it doesn’t, recheck your OIDC setup and the variables to avoid locking yourself out from administrator access.
+6. Complete the remaining setup steps
+
+> ⚠️ Important: Using a random username instead of your actual OIDC identity as the default user may result in no administrator access to your instance.
+
 ## Using Docker
 
 OIDC support in Docker is activated by the presence of a non-empty non-zero `OIDC_ENABLED` environment variable.
 
 > ℹ️ Only available in our default Debian image (not Alpine) for `x86_64` ([help welcome](https://github.com/FreshRSS/FreshRSS/issues/5722)).
 
-## The config is done with these environment variables
+## Configuration Environment Variables
 
 * `OIDC_ENABLED`: Activates OIDC support.
 * `OIDC_PROVIDER_METADATA_URL`: The config URL. Usually looks like: `<issuer>/.well-known/openid-configuration`
@@ -36,14 +53,6 @@ You may add additional custom configuration in a new `./FreshRSS/p/i/.htaccess`
 
 See our reference [Apache configuration](https://github.com/FreshRSS/FreshRSS/blob/edge/Docker/FreshRSS.Apache.conf) for more information.
 
-## Setup
-
-After being properly configured, OIDC support can be activated in FreshRSS.
-
-During a new FreshRSS install, the **HTTP Authentication Method** must be picked.
-
-After install, the method can be changed in *Administration > Authentication*. Note that this option will be greyed out if Apache is unable to read the `REMOTE_USER` variable.
-
 ## Identity Provider
 
 See specific instructions for: