Feature Request: Support for MDX Files (.mdx)

**Summary:** Extend Docling's document parsing capabilities to include `.mdx` (Markdown with JSX) files, alongside existing `.md` (Markdown) support.

**Background:** Docling currently excels at parsing a wide variety of document formats, including Markdown (`.md`). Markdown is a popular format for documentation, notes, and web content. However, the ecosystem is evolving, and **MDX** (Markdown + JSX) is gaining significant traction, particularly in modern documentation sites, component libraries, and interactive content.

MDX allows users to embed interactive React components and JSX expressions directly within Markdown content. This enables richer, more dynamic, and interactive documentation and content experiences compared to plain Markdown.