Refactor code structure for improved readability and maintainability

[Iamsdt]

This pull request updates the documentation and dependencies to switch the RTF file handler from pyrtf-ng to striprtf across the project. It also improves error handling for the DOC handler by detecting missing LibreOffice dependencies when using antiword. These changes ensure more reliable RTF extraction and clearer installation instructions.

Dependency and Documentation Updates:

• Changed the RTF handler dependency from pyrtf-ng to striprtf in pyproject.toml, and updated all related documentation files (docs/api.md, docs/index.md, docs/installation.md, docs/usage.md) to reflect this new requirement. [1] [2] [3] [4] [5]
• Updated log messages in textxtract/core/registry.py to reference striprtf instead of pyrtf-ng when the RTF handler is not installed.

Error Handling Improvements:

• Enhanced the DOC handler in textxtract/handlers/doc.py to detect missing LibreOffice dependencies when using antiword, and raise a FileNotFoundError to trigger fallback extraction methods.

Minor Codebase Cleanup:

• Removed an unused import of Path in textxtract/core/registry.py.

Python Version Documentation:

• Updated the tested Python versions in docs/installation.md to include Python 3.13.