Closes #11559: Implement config template rendering (#11769)

* WIP

* Add config_template field to Device

* Pre-fetch referenced templates

* Correct up_to_date callable

* Add config_template FK to Device

* Update & merge migrations

* Add config_template FK to Platform

* Add tagging support for ConfigTemplate

* Catch exceptions when rendering device templates in UI

* Refactor ConfigTemplate.render()

* Add support for returning plain text content

* Add ConfigTemplate model documentation

* Add feature documentation for config rendering

docs/features/configuration-rendering.md

@@ -0,0 +1,38 @@
+# Configuration Rendering
+
+One of the critical aspects of operating a network is ensuring that every network node is configured correctly. By leveraging configuration templates and [context data](./context-data.md), NetBox can render complete configuration files for each device on your network.
+
+```mermaid
+flowchart TD
+    ConfigContext & ConfigTemplate --> Config{{Rendered configuration}}
+
+click ConfigContext "../../models/extras/configcontext/"
+click ConfigTemplate "../../models/extras/configtemplate/"
+```
+
+## Configuration Templates
+
+Configuration templates are written in the [Jinja2 templating language](https://jinja.palletsprojects.com/), and may be automatically populated from remote data sources. Context data is applied to a template during rendering to output a complete configuration file. Below is an example template.
+
+```jinja2
+{% extends 'base.j2' %}
+
+{% block content %}
+    system {
+        host-name {{ device.name }};
+        domain-name example.com;
+        time-zone UTC;
+        authentication-order [ password radius ];
+        ntp {
+            {% for server in ntp_servers %}
+                server {{ server }};
+            {% endfor %}
+        }
+    }
+    {% for interface in device.interfaces.all() %}
+        {% include 'common/interface.j2' %}
+    {% endfor %}
+{% endblock %}
+```
+
+When rendered for a specific NetBox device, the template's `device` variable will be populated with the device instance, and `ntp_servers` will be pulled from the device's available context data. The resulting output will be a valid configuration segment that can be applied directly to a compatible network device.

docs/features/context-data.md

@@ -11,6 +11,8 @@ Configuration context data (or "config contexts" for short) is a powerful featur
 }
 ```
 
+Context data can be consumed by remote API clients, or it can be employed natively to render [configuration templates](./configuration-rendering.md).
+
 Config contexts can be computed for objects based on the following criteria:
 
 | Type          | Devices          | Virtual Machines |