conf.py (docs): Reorder the steps of the doc build

Since e121e267ad, the shared bindings matrix uses the stubs.
Therefore, we must build them!  This should fix the failure to build
the docs on readthedocs.org.

Neither @sommersoft nor I saw this locally since we had previously built
the stubs.  github CI didn't see it, because it manually builds the stubs
in an earlier step of the build process, and does not clean the tree
in between.

conf.py

@@ -42,6 +42,9 @@ master_doc = 'docs/index'
 # Grab the JSON values to use while building the module support matrix
 # in 'shared-bindings/index.rst'
 
+# The stubs must be built before we calculate the shared bindings matrix
+subprocess.check_output(["make", "stubs"])
+
 #modules_support_matrix = shared_bindings_matrix.support_matrix_excluded_boards()
 modules_support_matrix = shared_bindings_matrix.support_matrix_by_board()
 
@@ -77,7 +80,6 @@ source_suffix = {
     '.md': 'markdown',
 }
 
-subprocess.check_output(["make", "stubs"])
 extensions.append('autoapi.extension')
 
 autoapi_type = 'python'