docs: Add t4c instance documentation

docs/user/docs/settings/model-sources/t4c.md

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+<!--
+ ~ SPDX-FileCopyrightText: Copyright DB Netz AG and the capella-collab-manager contributors
+ ~ SPDX-License-Identifier: Apache-2.0
+ -->
+
+# Manage T4C instances
+
+## Define a T4C instance
+
+1.  Please navigate to `Profile` > `Settings`
+1.  Select `T4C` bewlow `Model sources`
+1.  You can see all existing instances (if any). To add a new instance, clock
+    on the "Add an instance" card. You have to enter the following information:
+    <!-- prettier-ignore -->
+    1. **Name**: Any name to identify the instance
+    1. **Capella version**: Capella version that corresponds to the instance
+    1. **License configuration**: License key of your license server
+    1. **Protocol**: Protocol that should be used to communicate between
+    capella sessions and the T4C server
+    1. **Host**: Hostname of the t4c server
+    1. **Port**, **CDO Port**, and **HTTP Port** Corresponding ports of your server
+    1. **License server API**: License server API url
+    1. **REST API**: REST API URL of the t4c server
+    1. **Username**: Username with access to the REST API, required for communication
+    with the REST API
+    1. **Password**: Password corresponding to username
+
+## Archive a T4C instance
+
+1.  Please navigate to `Profile` > `Settings`
+1.  Select `T4C` bewlow `Model sources`
+1.  Click on the instance that you want to archive
+1.  Click on the `Archive` button. When everything worked you should see a
+    messages stating "Instance updated: The instance _name_ is now archived"
+
+An archived instance can no longer be selected when creating a new t4c model
+and is highlighted with a gray background and an `Archived` tag in the bottom
+right in the t4c instance overview. Existing linked t4c models and all
+repositories corresponding to the archived instance will continue to work as
+before.