docs: install package in ReadTheDocs build (#3032)

Need to install ourselves so that we can introspect on version
using `setuptools_scm` in `docs/conf.py`.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

.readthedocs.yml

@@ -11,4 +11,7 @@ build:
 python:
   install:
     - requirements: docs-requirements.txt
-    - requirements: requirements.txt
+    - method: pip
+      path: .
+      extra_requirements:
+        - ssh