Reverse the order of install directives in RTD config

When RTD installs `.`, it uses `--upgrade-strategy eager`.  If this is
done after installing from `docs/requirements.txt`, packages may be
upgraded that shouldn't be; currently, `docutils` gets upgraded to 0.18,
which current versions of `Sphinx` can't work with.

.readthedocs.yaml

@@ -16,7 +16,8 @@ formats:
 python:
   # Mininum supported Python version
   version: "3.6"
+  # NB: install . first to avoid upgrading past docs/requirements.txt pins
   install:
-    - requirements: docs/requirements.txt
     - method: pip
       path: .
+    - requirements: docs/requirements.txt