Build from source: clarify dependency install step

[scpeters]

🦟 Bug fix

Clarify in response to this question: https://community.gazebosim.org/t/unable-to-install-gazebo-garden-in-ubuntu-20-04/2296

Summary

Clarify how to install dependencies when building from source. I've modified the harmonic folder doc only, so this is marked this as a draft; I will propagate the change to the other folders once the wording has been reviewed.

cc @tfoote

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] 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]

I amended the commit to apply the fix to all currently supported versions