updated readthedocs configuration

csst/.readthedocs-requirements.txt

+sphinx>=4.2.0
+astropy>=5.0.1
+numpy>=1.20.3
+scipy>=1.7.1
+matplotlib>=3.4.3

doc/preview.sh

+
+make clean html
+open ./build/html/index.html
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readthedocs.yml

+python:
+  # make sure joblib is installed in the virtualenv (it is imported in
+  # conf.py)
+  pip_install: true
+
+requirements_file: ..readthedocs-requirements.txt
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