Add AVL Tree and B Tree along with explanations.

[bhavinjawade]

@BassCoder2808
wrt reply to #8:

1. Write codes for AVL Tree and B Tree in python.
2. Write 2 markdowns explaining the data structures and the respective code.
3. Provide complexity analysis for various operations and along with use-cases of the data structures in the markdown.

[BassCoder2808]

Thanks for creating the issue!
I will create the above required files for the same.

[bhavinjawade]

Great. Assigned it to you.