docs: add awesome-css for Doxygen

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "libraries/Unity"]
 	path = libraries/Unity
 	url = https://github.com/ThrowTheSwitch/Unity
+[submodule "libraries/doxygen-awesome-css"]
+	path = libraries/doxygen-awesome-css
+	url = https://github.com/jothepro/doxygen-awesome-css.git

Doxyfile

@@ -290,7 +290,7 @@ ALIASES                =
 # members will be omitted, etc.
 # The default value is: NO.
 
-OPTIMIZE_OUTPUT_FOR_C  = NO
+OPTIMIZE_OUTPUT_FOR_C  = YES
 
 # Set the OPTIMIZE_OUTPUT_JAVA tag to YES if your project consists of Java or
 # Python sources only. Doxygen will then generate output that is more tailored
@@ -1600,7 +1600,7 @@ DISABLE_INDEX          = NO
 # The default value is: NO.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-GENERATE_TREEVIEW      = NO
+GENERATE_TREEVIEW      = YES #
 
 # When both GENERATE_TREEVIEW and DISABLE_INDEX are set to YES, then the
 # FULL_SIDEBAR option determines if the side bar is limited to only the treeview