Document usage of databus

[TomAFrench]

@guipublic has been working on adding support for the databus however we don't have any particular documentation about what this is and when it's useful.

We should write some technical documentation about:

• conceptually what functionality it allows in ACIR backends
• the types of applications where it would be useful
• how to write a Noir program which makes use of it

Documentation of how we change the compiled ACIR as a result of using the call_data and return_data visibility modifiers would also be good.

[guipublic]

I added the documentation to hackmd: https://hackmd.io/LB6oYnyoTF-YHMKvgH0vHg?view

[TomAFrench]

This isn't available to people outside of aztec labs however. Can you move the relevant information into public facing documentation?

[guipublic]

Should I add it to Noir Documentation?