Added the First time setup API documentation

Ticket: ENT-12185
Signed-off-by: Ihor Aleksandrychiev <ihor.aleksandrychiev@northern.tech>

Author: aleksandrychev

api/enterprise-api-ref/first-time-setup.markdown

@@ -0,0 +1,156 @@
+---
+layout: default
+title: Fist time setup API
+published: true
+---
+
+The First time setup API enables creation of the initial administrator user.
+This API is only accessible when the system hasn't been previously configured.
+Once setup is complete, this API becomes permanently inaccessible.
+
+## Generate setup code
+
+The setup code is generated during hub installation.
+If you need a new setup code, execute this command on your hub:
+
+```console
+sudo cf-hub --new-setup-code
+```
+
+**Note:** The setup code will expire after 1 hour.
+When you request a new setup code, it invalidates the previous one.
+
+## Check setup status
+
+First, check if the system has already been set up.
+This endpoint returns the current setup status of the system.
+
+**URI:** https://hub.example.com/api/setup/status
+
+**Method:** GET
+
+**Example request (curl):**
+
+```console
+curl -X GET \
+  https://hub.example.com/api/setup/status
+```
+
+**Successful response example:**
+
+```
+HTTP 200 Ok
+{
+    "is_setup_complete": false
+}
+```
+
+**Output:**
+
+* **is_setup_complete**
+  Boolean value indicating whether the system has been set up (true) or not (false)
+
+**Responses:**
+
+| HTTP response code        | Description                   |
+|---------------------------|-------------------------------|
+| 200 OK                    | Setup status check successful |
+| 500 Internal server error | Internal server error         |
+
+## Validate setup code
+
+Before completing setup, you must validate a setup code.
+This endpoint returns a session ID required for the setup complete API request.
+
+**URI:** https://hub.example.com/api/setup/code/validate
+
+**Method:** POST
+
+**Parameters:**
+
+* **code** *(string)*
+  The setup code provided during system initialization
+
+**Example request (curl):**
+
+```console
+curl -X POST \
+  https://hub.example.com/api/setup/code/validate \
+  --data-raw '{"code": "YOUR_SETUP_CODE"}'
+```
+
+**Successful response example:**
+
+```
+HTTP 200 Ok
+{
+    "session_id": "abcdef123456789....",
+    "valid": true
+}
+```
+
+**Output:**
+
+* **session_id**
+  The session ID to be used in the setup complete API request
+* **valid**
+  Boolean value indicating whether the provided code is valid
+
+**Responses:**
+
+| HTTP response code        | Description                   |
+|---------------------------|-------------------------------|
+| 200 OK                    | Code validation successful    |
+| 400 Bad request           | Invalid or missing code       |
+| 500 Internal server error | Internal server error         |
+
+## Complete setup
+
+This endpoint finalizes the setup process by creating the first administrator user.
+It requires a valid session ID obtained from the code validation step.
+
+**URI:** https://hub.example.com/api/setup/complete
+
+**Method:** POST
+
+**Headers:**
+
+* **Cf-Setup-Session-Id** *(string)*
+  Session ID obtained from the code validation step
+
+**Parameters:**
+
+* **username** *(string)*
+  Alphanumeric username for the administrator account
+* **password** *(string)*
+  Password for the administrator account
+* **email** *(string)*
+  Email address for the administrator account
+
+**Example request (curl):**
+
+```console
+curl -X POST \
+  https://hub.example.com/api/setup/complete \
+  --header 'Cf-Setup-Session-Id: abcdef123456789' \
+  --data-raw '{
+    "username": "admin",
+    "password": "SecurePassword123",
+    "email": "admin@example.loc"
+  }'
+```
+
+**Successful response example:**
+
+```
+HTTP 201 Created
+```
+
+**Responses:**
+
+| HTTP response code        | Description                                       |
+|---------------------------|---------------------------------------------------|
+| 201 Created               | Setup successfully completed                      |
+| 406 Not Acceptable        | Invalid session ID                                |
+| 400 Bad request           | Missing or invalid parameters                     |
+| 500 Internal server error | Internal server error                             |