Add Use Cases header menu to the documentation with the reference to CSign.io

[yshyn-iohk]

It will be a new section in the documentation with the real-life use case of the Identus platform.

The first real production use case is going to be CSign.io

Product website: https://csign.io/
Demo: https://discord.com/channels/@me/1369331560452456551/1369576140779421697

CsignLogos-1024.zip