Changed the readme file reference from README.rst to README.md and update project metadata

[pygarap]

This pull request updates the pyproject.toml configuration to improve project metadata, clarify Python version support, and enhance discoverability on package indexes. The main changes focus on updating documentation references, tightening Python compatibility, expanding project keywords and classifiers, and adding a homepage URL.

Project metadata and compatibility updates:

• Changed the readme file reference from README.rst to README.md for consistency with the repository.
• Updated the supported Python versions to require >=3.9,<3.15, clarifying compatibility and excluding unsupported future versions.

Discoverability and classification improvements:

• Expanded keywords to include "lexbor" and "modest" for better searchability.
• Significantly extended the list of classifiers to more accurately describe the project’s environment, operating systems, supported Python versions (including 3.14), and topics, improving visibility on PyPI.
• Added a Homepage URL to the project.urls section, making it easier for users to find the main project page.

Important

Right now, the README.md is not showing on https://pypi.org/project/selectolax/
This PR will fix this issue.

[rushter]

Thanks. Did not notice it got broken.