Document extra steps needed for updating to Nargo v0.11

[Savio-Sou]

Context

≥0.11.0 and until https://github.com/orgs/noir-lang/discussions/2709 or similar is addressed, Nargo lacks the ability to automatically check and upgrade proving backends installed on users' machines.

When an outdated / incompatible proving backend is used, Nargo would prompt the user with:

Error: The backend encountered an error

Location:
    tooling/nargo_cli/src/cli/mod.rs:90:5

Note: This does not affect new users installing Nargo for the first time, only those updating from older versions.

Solution

Add instructions for updating Nargo when encountering the error:

1. nargo backend uninstall acvm-backend-barretenberg
2. noirup
3. nargo compile (to initiate downloading and installing of the latest version of acvm-backend-barretenberg; might take minutes)

cc @TomAFrench would appreciate if you could verify the recommended steps.

[Savio-Sou]

Closed with 9d23ea7.