Add documentation for new opnsense integration (#10427)

* Add documentation for new opnsense component

This commit adds documentation for the opnsense device tracker which is
being added in home-assistant/home-assistant#26834.

* Update source/_components/opnsense.markdown

Co-Authored-By: Klaas Schoute <klaas_schoute@hotmail.com>

* Fixes from review comments

* More updates from code review

The previous updates in dc933232ab654d79a27cc0a16f8ca13b16196d46 around
the defaults were applied a bit too mechanically and the same problem
existed elsewhere in the configuration section. This commit also updates
those to fix the same issue there.

* ✏️ Tweaks

* Move documentation to new integrations location

* Remove LAN default for tracker_interfaces

Part of the review changes made to the PR adding this component was to
not set a default value for the tracker_interfaces to ["LAN"]. This
commit just updates the configuration section of the docs to reflect
this change.

* Update opnsense.markdown

Co-authored-by: Klaas Schoute <klaas_schoute@hotmail.com>
Co-authored-by: Franck Nijhof <frenck@frenck.nl>

source/_integrations/opnsense.markdown

@@ -0,0 +1,61 @@
+---
+title: "OPNsense"
+description: "Instructions on how to configure OPNsense integration"
+logo: opnsense.png
+ha_category:
+  - Hub
+  - Presence Detection
+ha_release: "0.106"
+---
+
+[OPNsense](https://opnsense.org/) is an open source HardenedBSD based firewall
+and routing platform. There is currently support for the following device types
+within Home Assistant:
+
+- [Presence Detection](#presence-detection)
+
+## Configuration
+
+To configure OPNsense integration with Home Assistant add the following section
+to your configuration.yaml:
+
+```yaml
+opnsense:
+  url: http://router/api
+  api_secret: API_SECRET
+  api_key: API_KEY
+```
+
+Where the `api_key` and `api_secret` values are acquired from your OPNsense
+router using the web interface. For more information on this procedure, refer
+to the OPNsense [documentation](https://docs.opnsense.org/development/how-tos/api.html#creating-keys).
+
+{% configuration %}
+url:
+  description: The URL for the OPNsense api endpoint of your router.
+  type: string
+  required: true
+api_key:
+  description: The API key used to authenticate with your OPNsense API endpoint.
+  type: string
+  required: true
+api_secret:
+  description: The API secret used to authenticate with your OPNsense API endpoint.
+  type: string
+  required: true
+verify_ssl:
+  description: Set to true to enable the validation of the OPNsense API's SSL.
+  type: boolean
+  required: false
+  default: false
+tracker_interfaces:
+  description: List of the OPNsense router's interfaces to use for tracking devices.
+  type: list
+  required: false
+  default: []
+{% endconfiguration %}
+
+
+## Presence detection
+
+This platform allows you to detect presence by looking at devices connected to an OPNsense router.