Add support for footnotes

Add a footnote number next to the text where the footnote appeared: `[1]`, and append at the end of the document all the footnotes:

```
[1]:  Nakamoto, Satoshi. Bitcoin: A Peer-to-Peer Electronic Cash System. https://bitcoin.org/bitcoin.pdf, 2008

[2]:  bitcoind peer discovery algorithms. https://en.bitcoin.it/wiki/Satoshi_Client_Node_Discovery

[3]:  https://getaddr.bitnodes.io/
```