*Encapsulated the Node class inside the DoublyLinkedList class
to keep it private and unaccessible from outside.
*Added a size attribute to the DoublyLinkedList class to keep
track of the number of elements in the list. This avoids iterating
over the list every time to calculate its length.
*Added proper error handling in the insert_at_nth and delete_at_nth methods to raise an IndexError when the index is out of range.

Describe your change:

• Add an algorithm?
• [✔️ ] Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• [✔️ ] I have read CONTRIBUTING.md.
• [✔️ ] This pull request is all my own work -- I have not plagiarized.
• [✔️ ] I know that pull requests will not be merged if they fail the automated tests.
• [✔️ ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• [ ✔️] All functions and variable names follow Python naming conventions.
• [✔️ ] All function parameters and return values are annotated with Python type hints.
• [✔️ ] All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".