Add aesara.as_symbolic to the documentation

[brandonwillard]

#743 introduced an aesara.as_symbolic dispatch function that converts any Python object into its symbolic Aesara equivalent. We should add this information to the documentation somewhere.

[rlouf]

@anirudhacharya asks:

I would also like to add this #746 in this PR itself. Can someone point me where in the documentation the as_symbolic function might go, would it be here - https://github.com/aesara-devs/aesara/blob/main/doc/library/graph/graph.rst

Originally posted in #1262 (comment)

[rlouf]

My first thought would be to add it to the API documentation main page as it is a general functionality available in the aesara namespace.