Add option to disable HTML escaping in Markdown serializer

[ceberam]

By default, the Markdown option escape_html is set to True in the MarkdownParams of the Markdownh serializer. That means that we perform HTML escaping when exporting to Markdown (e.g., for reserved characters like >, which gets serialized as >).
In certain cases we may want not to HTML escape and keep the original characters.
However, the serialization function export_to_markdown of DoclingDocument has no argument that can control the escape_html option and therefore the serialization is always done with escape_html set to its default True.

This is a request to expose escape_html to functions export_to_markdown and save_as_markdown.