Add doubly_to_singly() method to convert a doubly linked list to a singly linked list

[F-Blaze]

Describe your change:

• Add an algorithm

This pull request adds a new method doubly_to_singly() to the DoublyLinkedList class.

The method converts a doubly linked list into a singly linked list by creating a new singly linked list and copying the data from each node.

The implementation includes:

• type hints
• doctest examples
• a unit test in test_doubly_linked_list()

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.
• 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 automated testing.
• A Wikipedia reference has been included for the algorithm.