Install theme from pypi

Also updates the sphinx cap to match the one from the theme, which is more realistic. I haven't tried but doubt our doc build would still work with sphinx 1.5

Author: OriolAbril

conda-envs/environment-docs.yml

@@ -26,10 +26,10 @@ dependencies:
 - sphinx-copybutton
 - sphinx-design
 - sphinx-notfound-page
-- sphinx>=1.5
+- sphinx>=5
 - sphinxext-rediraffe
 - watermark
 - sphinx-remove-toctrees
 - pip:
-  - git+https://github.com/pymc-devs/pymc-sphinx-theme
+  - pymc-sphinx-theme==0.13
   - numdifftools>=0.9.40