add separate requirements file for docs, so we can pin docutils version (see sphinx-doc/sphinx#9727)

apdavison · apdavison · commit 61b6545ba20c · 2021-11-19T10:30:59.000+01:00
diff --git a/doc/requirements_docs.txt b/doc/requirements_docs.txt
@@ -0,0 +1,3 @@
+docutils<0.18
+numpy>=1.16.1
+quantities>=0.12.1
