manual: Convert cover SVG to a template file

We generate the previous SVG by doing sed substitutions to insert the
front cover and spine titles, plus the rear cover text. A follow-up
commit will generate the French and Spanish covers as well.

manual/cover/cover-en.sed

@@ -0,0 +1,13 @@
+s/__BACK_TEXT__/\
+Freedoom is an open-content First-Person\
+Shooter (FPS) game that is Free and Open\
+Source Software. Developed by volunteers\
+since 2001, Freedoom is made available\
+under the 3-clause BSD license.\
+\
+This manual gives a wealth of information,\
+including how to play the game, strategy,\
+tips and tricks and more./
+
+s/__FRONT_TITLE__/Game Manual/
+s/__SPINE_TITLE__/Freedoom Manual/