Relocate the "Setting up shell completions" doc page under Tooling

[Savio-Sou]

Problem

#6413 introduced the ability for Nargo to more tightly integrate with shells for auto-completion of commands in the CLI.

The doc page is currently categorized under "Getting Started".

Happy Case

Great to hear @noir-lang/developerrelations's thoughts if that's a good location for the doc page, or if we should relocate it somewhere else e.g. under "Tooling".