chore: Fix unconstrained function example

[jtriley-eth]

Description

This both fixes syntax errors and updates the unconstrained function example in the docs.

Summary

• fixes parenthesis mismatch
• replaces u72 with u64, as of noirc version 0.36.0+60c770f5f2594eea31ac75c852980edefa40d9eb
• updates compiler output to noirc version 0.36.0+60c770f5f2594eea31ac75c852980edefa40d9eb
• removes outdated optimization example

Additional Context

This block appears to come from a Discord post from July 2023, the u72 data type is rejected by the compiler as of this PR, so this was replaced with a u64. This of course changes the compiler output, but there was originally three steps of optimization: simple example, earlier cast, and unconstrained. The middle step, casting earlier, does not appear to reduce the number of ACIR opcodes anymore, I assume because of compiler optimization improvements, so this middle step was removed and wording was changed to accommodate this.

Also, can we remove the Discord link? If a reader is not in the Noir Discord server, the link leads to a blank page and offers no intuitive means to join the Discord server. If this is a means to drive Discord traffic, I would consider a link to join the server there rather than only in the page footer.

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

🚀 Deployed on https://67211e1bc350243274a37303--noir-docs.netlify.app

[github-actions]

FYI @noir-lang/developerrelations on Noir doc changes.