fix: restore floating-item footnotes in markdown/html exports

[M-Hassan-Raza]

Closes #496.

This restores floating-item footnotes in Markdown export and fixes the same shared path for HTML and chunking.

The main change is that footnotes now go through the normal text serializer, so formatting and hyperlinks are preserved instead of being flattened to plain text. I also tightened the HTML output a bit so footnotes render as a grouped note block alongside the related table or figure.

Tests:

• uv run pytest -q test/test_serialization.py test/test_docling_doc.py
• uv run --extra chunking pytest -q test/test_hierarchical_chunker.py test/test_hybrid_chunker.py
• uv run ruff check docling_core/transforms/serializer/common.py docling_core/transforms/serializer/markdown.py docling_core/transforms/serializer/html.py docling_core/transforms/chunker/hierarchical_chunker.py test/test_serialization.py test/test_hierarchical_chunker.py

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