doc: improve readme

[lhoupert]

This pull request updates the README.md to clarify the purpose and value proposition of the GitHub Action, focusing on when and why to use it. The changes make the documentation more concise and user-focused, replacing a feature comparison table with a clearer explanation of the action’s unique benefits.

Documentation improvements:

• Rewrote the introduction to clarify that the action is a wrapper for both Bandit and pip-audit, and simplified the language for better readability.
• Replaced the feature comparison table with a narrative explanation of scenarios where this action is useful, emphasizing its unified reporting and workflow enhancements.
• Listed the main added features: unified PR comment, workflow step summary, configurable blocking on fixable vulnerabilities, and automatic requirements export for different package managers.