docs: re-home mkdocs sources within this repo (#2149)

docs/Dockerfile

@@ -0,0 +1,10 @@
+FROM python:3.8
+
+RUN pip install --upgrade pip
+
+WORKDIR /mkdocs
+
+RUN --mount=target=/build/requirements.txt,source=docs/requirements.txt \
+    pip install -r /build/requirements.txt
+
+ENTRYPOINT ["mkdocs"]