Maximum length for the markdown exporter

When stuffing content to LLMs you want to get the biggest chunk that fits in the maximum context window.

A trivial implementation could simply do `content_md = doc.export_to_markdown()[:MAX_LEN]`. However, this could break sentences, paragraphs, tables, etc.

If we introduce a `max_size` in the `export_to_markdown()` function, we could control it by adding a document item only if it doesn't go above the limit.