Support deleting and modifying existing footnotes. Depends on #1-#3.

• footnote.delete() — removes the footnote from footnotes.xml AND removes the w:footnoteReference from the document body
• Modify footnote content via footnote.paragraphs (already supported if Paragraph API works)
• footnote.clear() — remove all content paragraphs, add a single empty one
• Handle renumbering if needed (or just leave gaps in IDs)

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