Update .readthedocs.yml (#354)

* Update .readthedocs.yml

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.readthedocs.yml

@@ -1,9 +1,24 @@
 # Copyright 2021-2023 Axel Huebl
 #
-# This file is part of ImpactX.
+# This file is part of ImpactX
 #
 # License: BSD-3-Clause-LBNL
 
-requirements_file: docs/requirements.txt
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
+sphinx:
+   configuration: docs/source/conf.py
+
+python:
+   install:
+   - requirements: docs/requirements.txt
 
 formats:
+  - htmlzip
+#  - pdf
+#  - epub