boards: v2: Add documentation index for converted boards

Adds documentation pages for nios2-converted boards

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

boards/index.rst

@@ -29,6 +29,7 @@ available under :zephyr_file:`doc/templates/board.tmpl`.
    shields/index.rst
 
    v2/adafruit/index.rst
+   v2/altera/index.rst
    v2/arm/index.rst
    v2/gd/index.rst
    v2/raspberry_pi/index.rst

boards/v2/altera/index.rst

@@ -0,0 +1,10 @@
+.. _boards-altera:
+
+Altera Corporation
+##################
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   **/*