Add Infra-Server 15 and re-organize for easier use

[Stromweld]

Description

Added Infra-Server 15 to Matrix and re-arranged Upgrade sections so the 'General upgrade procedure' section is first to help eliminate confusion.

Check List

• [ ] New functionality includes tests
• [ ] All buildkite tests pass
• [ ] Full omnibus build and tests in buildkite pass
• [ ] All commits have been signed-off for the Developer Certificate of Origin. See https://github.com/chef/chef/blob/main/CONTRIBUTING.md#developer-certification-of-origin-dco
• [ ] PR title is a worthy inclusion in the CHANGELOG

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