docs: rework the README to the org layout and fix two false statements

[shenxianpeng]

Part of aligning the READMEs across the org (same layout as cpp-linter/cpp-linter-hooks#289).

What changed

Header

• Badges: ci (build.yml) and part of cpp-linter.
• Removed the <div>, the badges inside the H1, the Lint badge and the emoji hub badge.
• There is no release badge: asdf plugin add clones main, so nobody installs a release tag.
• Sections were H1 headings; they are H2 now.

False today, now fixed

• asdf set comment: it said the command writes ~/.tool-versions. Since asdf 0.16 it writes ./.tool-versions; -u writes the home file. The Quick start now says it needs asdf 0.16 or later.
• macOS de-quarantine: the plugin never offers it. lib/utils.bash checks for macosx, but the platform is macos, so ASDF_CLANG_TOOLS_MACOS_DEQUARANTINE does nothing. curl does not set the quarantine attribute, and the binaries ran with no Gatekeeper prompt. The variable is removed from the README, which now says this.

Clarified, each tested

• The tools are not in asdf's plugin list, so the URL is required.
• The plugin name must be the tool name: with clang-tools as the name, install fails.
• Versions are LLVM majors: asdf install clang-format 21.1.0 fails.
• The bare clang-format example read stdin and waited; it now runs clang-format --version.
• GITHUB_TOKEN is now listed: the scripts send it with the release API request.

Removed

• The Windows line: asdf has no Windows release, and plugin CI has no Windows runner.
• The "make sure you trust that repository" line and duplicates.

Checked

• The Quick start ran as written with asdf 0.20.2 (latest) in a throwaway install. clang-format --version printed the LLVM 23 build.
• All links return 200.
• The repo's pre-commit hooks pass.

Not in this PR

• The dead de-quarantine block in lib/utils.bash and its env var in build.yml.
• The asdf plugin test clang-tools ... commands in CONTRIBUTING.md, which fail.