[Resolves #1327] Setup readthedocs (#1404)

Setup readthedocs to generate and host Sceptre docs from
readthedocs.org[1].  It will update the docs on every change
and it will keep and host the entire history of releases.

[1] https://docs.readthedocs.io/en/stable/build-customization.html#install-dependencies-with-poetry

.readthedocs.yaml

@@ -0,0 +1,20 @@
+version: 2
+
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.10"
+  jobs:
+    post_create_environment:
+      # Install poetry
+      # https://python-poetry.org/docs/#installing-manually
+      - pip install poetry
+      # Tell poetry to not use a virtual environment
+      - poetry config virtualenvs.create false
+    post_install:
+      # Install dependencies with extras dependency group
+      # https://python-poetry.org/docs/managing-dependencies/#dependency-groups
+      - poetry install --all-extras
+
+sphinx:
+  configuration: docs/_source/conf.py