Improve install instructions for macOS

[rgov]

Summary

Several improvements to the install instructions for macOS including,

• Using curl instead of wget, which is not built-in.
• Unifying environment variable settings between Intel and Apple Silicon Macs.
• Instructing the user how to install the Xcode Command Line Tools.
• Directing users to the Homebrew documentation rather than duplicating it.
• Prioritizing Zsh (the default shell) over Bash.
• Partially addresses #106.

I've only made the changes to Harmonic so far, but once approved they can be applied to Ionic as well.

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [x] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[scpeters]

@j-rivero I'll let you merge this since it may conflict with https://github.com/gazebosim/docs/pull/451