[DOC] Document files

The file https://github.com/bitcoin/bitcoin/blob/master/doc/files.md documents files used by Bitcoin Core.

Missing description for:

```
banlist.dat
[ bitcoin.conf ]
bitcoind.pid
[ db.log ]
[ debug.log ]
fee_estimates.dat
```
