Use a version placeholder in doxyfile

The version should be obtained from AssemblyInfo
while generating documentation via apidoc script.

DEVSIX-3466

Author: ars18wrw

doxyfile

@@ -32,7 +32,7 @@ DOXYFILE_ENCODING      = UTF-8
 # title of most generated pages and in a few other places.
 # The default value is: My Project.
 
-PROJECT_NAME           = "iText 7 7.1.8 API"
+PROJECT_NAME           = "iText 7 VERSION_PLACEHOLDER API"
 
 # The PROJECT_NUMBER tag can be used to enter a project or revision number. This
 # could be handy for archiving the generated documentation or if some version