feat: Enable markdown text formatting for docx

[SimJeg]

Hi,

This PR adds markdown text formatting for docx documents (italic, bold, underline and hyperlinks). I included a new tests/data/docx/unit_test_formatting.docx document to illustrate it. Using the latest docling main the output of export_to_markdown is:

italic
bold
underline
hyperlink
italic and bold hyperlink
italic bold underline and hyperlink on the same line

with this PR it becomes:

italic
bold
underline
hyperlink
italic and bold hyperlink
italic bold underline and hyperlink on the same line

[mergify]

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[SimJeg]

Note: for underline I used the <u> / </u> tags that are not rendered on GitHub 😅