Tweaks to 'Source Installation on Ubuntu' page

[chen-harrison]

Summary

• Requested by @azeey from docs: install_ubuntu_src gazebo_test_cases#2257
• Include sudo apt update before install required packages
• Add -y to 'sudo apt install' command for consistency with the rest of the documentation
• More informative warning section about RAM usage above colcon build commands

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)
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[chen-harrison]

I think these changes are also relevant for the equivalent page on previous versions of Gazebo, not just Jetty, so let me know if you'd prefer to have these changes applied to the equivalent pages for Ionic, Harmonic, etc.

[azeey]

Thanks for the PR! And yes, please update the ionic and harmonic folders as well.