Improved Footnote Serialization in `MarkdownDocSerializer`

### Requested feature
Currently, footnotes are serialized as part of `MarkdownDocSerializer` more or less as-is:

Serialized as:

```
5 https://github.com/tesseract-ocr/tesseract

6 https://github.com/VikParuchuri/surya

7 https://github.com/lukas-blecher/LaTeX-OCR
```

### Alternatives
For downstream LLM-based applications it would be helpful if footnotes were serialized as actual footnotes in Markdown Syntax for the LLM to indentify them as footnotes (and not as a numbered list, for example).

```
^[5 https://github.com/tesseract-ocr/tesseract]

^[6 https://github.com/VikParuchuri/surya]

^[7 https://github.com/lukas-blecher/LaTeX-OCR]
```