DOC: include description for the section with vectorize

[williambdean]

Issue with current documentation:

https://pytensor.readthedocs.io/en/latest/library/tensor/functional.html doesn't have H1 which shows up with in google search

This also shows up in the sidebar without a description:

Idea or request for content:

Add some type of title for the function section. Maybe here:

pytensor/doc/library/tensor/functional.rst

Lines 1 to 2 in f867ecb

	.. automodule:: pytensor.tensor.functional
	:members: vectorize

[ricardoV94]

Sure @williambdean