Add a Theoretical Markdown File for Big-O Notation

The project would benefit from a thorough Big-O Notation explanation.

Create a new markdown file BigONotation.md.

Explain:

- What Big-O Notation is.

- Different complexity classes (O(1), O(n), O(log n), O(n²), etc.).

- How to calculate time and space complexity of an algorithm.

- Common examples of algorithms with their Big-O classifications.

- Link back to relevant sections in the project (e.g., Sorting Algorithms, Graph Algorithms).

- Provide a table summarizing time/space complexity of data structures and algorithms in the project.

- Include visual aids (ASCII diagrams or links to visual resources).

Update the "Table of Contents" in the README.