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.

Create BigONotation.md and start by reviewing the README Table of Contents and the referenced Sorting Algorithms and Graph Algorithms sections. Done means the file explains Big-O and complexity calculation, covers the requested classes and examples, includes the summary table and visual aids or links, and the README Table of Contents links to it.

Written by the indexing model from the issue text.