docs: Add interactive Google Colab tutorial and quickstart notebook

[umran666]

Description

To make it effortless for researchers and developers to test UniqToken without local installation, we need a clean, interactive Google Colab notebook.

Scope of Work

Create a new notebook at notebooks/quickstart.ipynb demonstrating:

1. Installing UniqToken and building the Rust native core.
2. Training a custom Unigram tokenizer on a sample multilingual dataset.
3. Visualizing token compression (Bytes/Token) vs standard Tiktoken.
4. Exporting the trained model directly to Hugging Face tokenizer.json.

Getting Started

• Folder: notebooks/
• Add an "Open in Colab" badge to README.md.