Update references of the main branch to gz-cmake5

[azeey]

Summary

Part of gazebosim/gz-jetty#19

Also fixes a few typos and broken links.

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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