Install theme from pypi (pymc-devs#534)

* Update theme version and install from pip

Author: OriolAbril

requirements-docs.txt

@@ -1,6 +1,6 @@
 myst-nb
 sphinx==5.0.2 # see https://github.com/pymc-devs/pymc-examples/issues/409
-git+https://github.com/pymc-devs/pymc-sphinx-theme
+pymc-sphinx-theme==0.1
 sphinx-design
 sphinx-copybutton
 sphinxcontrib-bibtex