doc: make sure section depths are consecutive

skipping heading levels (eg from # to ###, or starting at ###) is legal
in pandoc, but not in nixos-render-docs. pandoc acts as though section
levels *were* consecutive, nixos-render-docs prefers to tell people not
to do that kind of thing because it can make documents more fragile.

doc/hooks/python.section.md

@@ -1,4 +1,3 @@
-
-### Python {#setup-hook-python}
+# Python {#setup-hook-python}
 
 Adds the `lib/${python.libPrefix}/site-packages` subdirectory of each build input to the `PYTHONPATH` environment variable.