Enhance documentation with signing knowledge

@nguyer raised on the last community call that in the Discord channel he was to frequently explain how signing works in Firefly and suggested that we should have a section in the documentation to explain in more detail how signing works, how to add keys, etc...

This issue is to start that discussion on what to add to the documentation, a few examples from discord:
- https://discord.com/channels/905194001349627914/1206875106429640724/1206956787413295104
- https://discord.com/channels/905194001349627914/1198761803849670926/1198985722229702749
- https://discord.com/channels/905194001349627914/1197148445417414737/1197212017182916810

It would be enhancing the documentation at https://github.com/hyperledger/firefly-signer or adding a page to the main Firefly docs