Maximum length for the markdown exporter

[dolfim-ibm]

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.