Reorder installation in .readthedocs.yaml (#825)

* 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.

* Update .readthedocs.yaml

Co-authored-by: Brian Rutledge <brian@bhrutledge.com>

.readthedocs.yaml

@@ -16,7 +16,10 @@ formats:
 python:
   # Mininum supported Python version
   version: "3.6"
+  # Install twine first, because RTD uses `--upgrade-strategy eager`,
+  # which installs the latest version of docutils via readme_renderer.
+  # However, Sphinx 4.2.0 requires docutils>=0.14,<0.18.
   install:
-    - requirements: docs/requirements.txt
     - method: pip
       path: .
+    - requirements: docs/requirements.txt