Document nargo backend

[Savio-Sou]

As of noir-lang/noir#2632, the nargo backend commands are now unhidden and Nargo users can now install and switch between different proving backends.

noir-lang/noir#2552:

• nargo backend ls: lists all of the directories in ~/.nargo/backends
• nargo backend use <name>: switches backend

noir-lang/noir#2575:

• nargo backend current: prints the name of the currently active backend
• nargo backend install <name> <url>: installs a backend from under the name
• nargo backend uninstall <name>: uninstalls a backend with the the name