docs: update README with improved TOON description and installation i…

[smortezah]

This pull request updates the README.md to clarify the purpose and usage of the TOON format, improves onboarding instructions, and reorganizes documentation for better readability. The most important changes include clearer explanations of TOON, a new installation section, improved example formatting, and a more logical structure for resources and usage.

Documentation and explanation improvements:

• Clarified that TOON is a serialization format intended for LLM input, not output, and added a tip comparing TOON to JSON as a translation layer.

Onboarding and installation:

• Added a new "Installation" section with instructions for installing the toon-format package using uv.

Example formatting:

• Specified the code block language as toon for the TOON example to improve syntax highlighting and readability.

Usage and resources organization:

• Updated the usage example to use a string for data, added an assertion for round-trip encoding/decoding, and moved the "Resources" section to follow usage instructions for better flow.

Resource links:

• Reformatted resource links as a bullet list for clarity and accessibility.

[johannschopplich]

I love these small, incremental README fixes! Could you please re-open once the repo is ready? The first version is currently in development. Let's wait until #6 is resolved and the team has formed. Please re-open later. Thank you for your understanding!