Re-format CHANGELOG.md

[ybiquitous]

Which issue, if any, is this issue related to?

Closes #3901

Is there anything in the PR that needs further explanation?

Changes summary:

• Introduce a new 1st level heading
• Change headings (including each version number) from 1st level to 2nd level
• Add GitHub compare link between tags
• Update descriptions related to CHANGELOG.md in developer guide

I wrote the following Ruby script for this task:

original_content = File.read("CHANGELOG.md")

versions = original_content.lines.
             select { |line| line.start_with?("#") }.
             map { |line| line.match(/# ([0-9\.-]+)\b/)[1] }

compare_urls = versions.map.with_index do |version, index|
  prev_version = versions[index + 1]
  next unless prev_version

  if prev_version == "0.1.0"
    prev_version = "v0.1.0"
  end

  if prev_version == "3.0.0-3.0.2"
    "[#{version}]: https://github.com/stylelint/stylelint/compare/3.0.2...#{version}"
  elsif version == "3.0.0-3.0.2"
    "[#{version}]: https://github.com/stylelint/stylelint/compare/2.3.7...3.0.2"
  else
    "[#{version}]: https://github.com/stylelint/stylelint/compare/#{prev_version}...#{version}"
  end
end.compact

updated_content = original_content.gsub(/^# ([0-9\.-]+)/m) do |_|
  if $1 == versions.last
    "## #{$1}"
  else
    "## [#{$1}][]"
  end
end

updated_content = <<~EOF
# Changelog

All notable changes to this project are documented in this file.

## [Head][]

#{updated_content}
[Head]: https://github.com/stylelint/stylelint/compare/#{versions.first}...HEAD
#{compare_urls.join("\n")}
EOF

File.write("CHANGELOG.md", updated_content)

[jeddy3]

LGTM!

Nice work with the script.

We'll only know if this new format works correctly with npmpub when we next release. Fingers crossed...