feat: Add support for Markdown files

[ali-sedaghatbaf]

Description

This pull request introduces a major refactor and improvement to the document loading system in the experimental SimpleKG pipeline, expanding support for Markdown files and standardizing terminology and interfaces across the codebase and documentation. The changes include replacing the legacy from_pdf and pdf_loader parameters with the more general from_file and file_loader, updating documentation and examples accordingly, and adding support for Markdown input. Backwards compatibility is preserved with deprecation warnings for old parameter names.

• The SimpleKG pipeline now supports Markdown (.md, .markdown) files in addition to PDFs when using the default FileLoader (CHANGELOG.md).
• The parameters and configuration keys from_pdf and pdf_loader are replaced by from_file and file_loader throughout the codebase, configuration files, and documentation. Legacy names are still accepted with a deprecation warning (user_guide_kg_builder.rst, README.md, multiple example scripts and config files).
• The PdfLoader and related classes are moved from pdf_loader to the new data_loader module, which also introduces MarkdownLoader and a unified FileLoader. All relevant imports and documentation are updated (api.rst, example scripts).
• The LoadedDocument type replaces PdfDocument as the standard output for data loaders, and documentation is updated to reflect this (types.rst, example scripts).
• A new exception, UnsupportedDocumentFormatError, is introduced for unsupported file types in the document loader (exceptions.py).

Type of Change

• New feature
• Bug fix
• Breaking change
• Documentation update
• Project configuration change

Complexity

Complexity: low

How Has This Been Tested?

• Unit tests
• E2E tests
• Manual tests

Checklist

The following requirements should have been met (depending on the changes in the branch):

• Documentation has been updated
• Unit tests have been updated
• E2E tests have been updated
• Examples have been updated
• New files have copyright header
• CLA (https://neo4j.com/developer/cla/) has been signed
• CHANGELOG.md updated if appropriate

[jonnylaw]

Good stuff 🚀

A few missing tests and the MarkdownLoader currently returns PdfLoaderError.