docs: generating Doxygen locally and new Sourceforge URL

The documentation for getting game data was slightly out of date so this
commit updates the new process for getting a snapshot.

As the online version of Doxygen code docs does not seem to be online,
instructions have been added for generating the docs locally. Doxyfile
was also updated using Doxygen's automated update: `doxygen -u`.

Added the doxygen folder to *.gitignore*. I am not sure if this breaks
any pipelines used by the project.

Author: Matthew Cantelon

.gitignore

@@ -2,6 +2,8 @@
 *.dll
 *.exe
 *.out
+*.swp
+*.bak
 .DS_Store
 .sconf_temp/
 .sconsign.dblite
@@ -11,6 +13,7 @@ obj
 build
 data
 vdrift
+doxygen
 vdrift-mac
 vdrift-win
 vdrift.cfg