Added "What is Aesara" section to documentation

[Mount-Blanc]

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Issue: #967

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[ricardoV94]

I am not sure we want this in the README. Right now the README is pretty succint and I think that's a good thing.

Perhaps under Aesara at a glance? https://aesara.readthedocs.io/en/latest/introduction.html

https://github.com/aesara-devs/aesara/blob/main/doc/introduction.rst

We already had a section on the difference between Aesara and Numpy

It might also require some small tuning to not sound out of place or repetitive in the context of the rest of the docs.

[brandonwillard]

Looks good, thanks!