add pyvista to sites using the theme (#418)

Author: akaszynski

README.md

@@ -22,6 +22,7 @@ Sites that are using this theme:
 - MegEngine: https://megengine.org.cn/doc/stable/zh/
 - Fairlearn: https://fairlearn.github.io/main/quickstart.html
 - NetworkX: https://networkx.org/documentation/latest/
+- PyVista: https://docs.pyvista.org
 
 
 ## Installation and usage