Revert "Delete requirements.txt"

This reverts commit 9aa4f06.

Doc/requirements.txt

@@ -0,0 +1,12 @@
+# Requirements to build the Python documentation
+
+# Sphinx version is pinned so that new versions that introduce new warnings
+# won't suddenly cause build failures. Updating the version is fine as long
+# as no warnings are raised by doing so.
+sphinx==4.2.0
+
+blurb
+
+# The theme used by the documentation is stored separately, so we need
+# to install that as well.
+python-docs-theme