Support reading footnotes from existing documents. Depends on #1, #2.

• document.footnotes iterates all user footnotes (skipping separator/continuation at id 0,1)
• Each Footnote exposes .paragraphs with full Paragraph API (runs, formatting, text)
• Footnote.text property returns concatenated text of all paragraphs
• Handle documents with no footnotes.xml part gracefully (empty collection)

Files: docx/footnotes.py, docx/parts/footnotes.py