docs: Add more information about formats

[vitonsky]

I've read this https://github.com/netheril96/securefs/blob/master/docs/design.md#difference-between-1-2-and-3 and it's not so clear to understand. Could you provide more details about each format?

I expect description with sub-headers like this

Formats

Format 1

Some info about format in details

Format 2

Some info about format in details