[doc] [practical exercises] Fix file links

doc/Doxyfile.extra.in

@@ -317,6 +317,9 @@ FILE_PATTERNS          = *.cc *.cpp *.h *.hpp *.hh *.dox *.md *.py
 
 EXCLUDE                = @CMAKE_SOURCE_DIR@/doc/d-practical-exercises/src
 
+# Put them back for inclusion as whole files
+HTML_EXTRA_FILES       += @CMAKE_SOURCE_DIR@/doc/d-practical-exercises/src
+
 # The EXCLUDE_SYMLINKS tag can be used to select whether or not files or
 # directories that are symbolic links (a Unix file system feature) are excluded
 # from the input.