Copy static x3d files before running sphinx-build

This commit resolves an issue where Sphinx will not copy static X3D files from the source to the build directory before loading extensions, which results in a FileNotFoundError and causes the build process to exit. It is documented in Sphinx issues #2090 and #1810 that this is expected behavior.

doc/Makefile

@@ -7,6 +7,7 @@ SPHINXOPTS    ?=
 SPHINXBUILD   ?= sphinx-build
 SOURCEDIR     = source
 BUILDDIR      = build
+STATICDIR     = _static
 
 # Put it first so that "make" without argument is like "make help".
 help:
@@ -16,5 +17,12 @@ help:
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
+# Note:
+#	Second line ensures that there is a build x3d folder
+# 	Third line copies the X3D files from _static (Sphinx doesn't copy them correctly)
+#	Fourth line runs the sphinx build command
 %: Makefile
+	@echo "Creating ./$(BUILDDIR)/$@/$(STATICDIR)/x3d..."; 
+	@mkdir -p "$(BUILDDIR)/$@/$(STATICDIR)/x3d"
+	@cp -r "./$(SOURCEDIR)/$(STATICDIR)/x3d" "./$(BUILDDIR)/$@/$(STATICDIR)"
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)