Improve installation instructions (backport #1490)

[mergify[bot]]

🦟 Bug fix

Improves installation instructions in README

Summary

• Refer to https://brew.sh/ instead of duplicating the brew installation command.
• List cmake variables in a markdown table.
• Combine Ubuntu and macOS build-from-source instructions (https://github.com/gazebosim/sdformat/commit/85e50675bd7b975ca9130fbbd8832a0ae6105593)

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)
• [X] codecheck passed (See contributing)
• [X] 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.

---

This is an automatic backport of pull request #1490 done by Mergify.

[mergify[bot]]

Cherry-pick of 22684cbe9144f9cf15e2df7dfa55457266caca44 has failed:

On branch mergify/bp/sdf14/pr-1490
Your branch is up to date with 'origin/sdf14'.

You are currently cherry-picking commit 22684cbe.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   README.md

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally