Get doxygen working again.

This also enables MarkDeep for the comments, but I haven't added
any examples of that yet.

Screenshots:
  https://screenshot.googleplex.com/mUCgtAesDWt.png
  https://screenshot.googleplex.com/S1p4Dhpn6KW.png

Bug: skia:
Change-Id: I4f645ef1050d26c7a4660a1fc28a8d41f085f4f8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/198241
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>
diff --git a/tools/doxygen/ProdDoxyfile b/tools/doxygen/ProdDoxyfile
new file mode 100644
index 0000000..afcacff
--- /dev/null
+++ b/tools/doxygen/ProdDoxyfile
@@ -0,0 +1,5 @@
+# This config is used to generate the docs under continuous integration.
+
+@INCLUDE = Doxyfile
+
+OUTPUT_DIRECTORY       = /workspace/__doxygen_staging