add ip_fortress svg file

I have to put and display png as firefox have a bug
it does not abble to draw arrow from inkscape

source/_static/unix/images/ip_fortress.svg

@@ -9,6 +9,9 @@
    id="svg5"
    inkscape:version="1.1.2 (0a00cf5339, 2022-02-04)"
    sodipodi:docname="ip_fortress.svg"
+   inkscape:export-filename="/home/bneron/Projects/Cours/Given/Unix_tunis/src/unix_tunis/source/_static/unix/images/ip_fortress.png"
+   inkscape:export-xdpi="96"
+   inkscape:export-ydpi="96"
    xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
    xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
    xmlns="http://www.w3.org/2000/svg"
@@ -28,12 +31,12 @@
      inkscape:snap-global="false"
      inkscape:zoom="1.59"
      inkscape:cx="116.3522"
-     inkscape:cy="390.56604"
-     inkscape:window-width="3840"
-     inkscape:window-height="2088"
-     inkscape:window-x="0"
+     inkscape:cy="390.8805"
+     inkscape:window-width="3120"
+     inkscape:window-height="2077"
+     inkscape:window-x="720"
      inkscape:window-y="0"
-     inkscape:window-maximized="1"
+     inkscape:window-maximized="0"
      inkscape:current-layer="layer1"
      fit-margin-left="1"
      fit-margin-top="1"