Documentation: Add internal-api-html placeholder

Mimicking the existing api-html placeholder, add a placeholder for the
internal version of the Doxygen generated API documentation so that the
website can generate hyperlinks to it properly.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Documentation/documentation-contents.rst

@@ -3,6 +3,7 @@
 .. container:: documentation-nav
 
    * :doc:`/api-html/index`
+   * :doc:`/internal-api-html/index`
    * :doc:`/camera-sensor-model`
    * :doc:`/code-of-conduct`
    * :doc:`/coding-style`

Documentation/internal-api-html/index.rst

@@ -0,0 +1,8 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+.. _internal-api:
+
+Internal API Reference
+======================
+
+:: Placeholder for Doxygen documentation

Documentation/introduction.rst

@@ -13,6 +13,7 @@ Documentation
    :hidden:
 
    API <api-html/index>
+   Internal API <internal-api-html/index>
 
 What is libcamera?
 ==================