Add more aggregation examples in our PyMongo docs

We received feedback from our docs user in the recent study conducted that we don't have enough code examples in our docs. One of them was related to aggregation. 

"

But these examples aren’t as useful as they could be because I’m a python developer. I write python. So I need examples for aggregations in python. Where do I find those?

… I navigate to drivers.

… I select Python.

… I select Pymongo

… I select Tutorial (this is the wrong place!... but i don’t know that so i look around). 

… I select index, aggregation is not listed …… %I#E%#EIU%!

… I search the pymongo docs for aggregation

… I find aggregation examples

… no. there’s only one example! And it doesn’t show how to do literally anything omg. I have now looked through 9 different areas, all of which are at least a page of text.  

We cannot be a developer data platform with documentation that has a lack of code examples. 

I google. I find this [*https://www.mongodb.com/developer/languages/python/python-quickstart-aggregation/*]

"

To Do: Add more aggregation examples in our pymongo docs

Task for shubham.ranjan@mongodb.com : Check if we need to add any other code examples.