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

@@ -4,11 +4,14 @@ sphinx:
   configuration: docs/conf.py
 
 build:
-    os: ubuntu-20.04
-    tools:
-        python: '3.10'
+  os: ubuntu-20.04
+  tools:
+    python: '3.10'
 
 python:
   install:
     - requirements: docs-requirements.txt
-    - requirements: requirements.txt
+    - method: pip
+      path: .
+      extra_requirements:
+        - ssh