Backport PR pandas-dev#45923: DOC: add Python 3.10 to doc

Author: eshirvana

doc/source/getting_started/install.rst

@@ -20,7 +20,7 @@ Instructions for installing from source,
 Python version support
 ----------------------
 
-Officially Python 3.8, and 3.9.
+Officially Python 3.8, 3.9 and 3.10.
 
 Installing pandas
 -----------------