Use NUMBA_DISABLE_JIT when building docs

As seen in https://github.com/empymod/emg3d/blob/master/docs/Makefile

docs/Makefile

@@ -1,6 +1,9 @@
 # Minimal makefile for Sphinx documentation
 #
 
+# Disable numba, so jitted functions are nicely documented.
+export NUMBA_DISABLE_JIT=1
+
 # You can set these variables from the command line.
 SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
@@ -16,4 +19,4 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)