docs: add C++20 compiler support matrix across Linux/macOS/Windows

[Easton97-Jens]

Motivation

• Provide an auditable, GitHub-ready reference for C++20 compiler and distribution support to make feature-level verification reproducible.
• Consolidate authoritative links for GCC, Clang/LLVM, MSVC, and cppreference and clarify differences across OS toolchains and Linux distribution packaging.
• Clarify CentOS/CentOS Stream relationship to RHEL and the CentOS Linux EOL status using only official Red Hat/CentOS sources.

Description

• Add a new documentation file at doc/cpp20-compiler-support-matrix.md containing the requested titled document and the specified section structure.
• Include official per-compiler references, a distribution table covering Ubuntu, Debian, Fedora, CentOS, CentOS Stream, Rocky, AlmaLinux, openSUSE, Arch, Alpine, Gentoo, and NixOS, and a historical GCC support summary.
• Add macOS and Windows sections explaining Apple Clang vs upstream Clang/GCC, Xcode/Homebrew context, and native vs compatibility Windows toolchains (MSVC, MinGW-w64, MSYS2, Cygwin, WSL).
• Provide reproducible verification commands for g++, clang++, cl, __cplusplus macro checks, and include only authoritative source links inline.

Testing

• Created and committed the new file using git add doc/cpp20-compiler-support-matrix.md and git commit -m "docs: add auditable C++20 compiler support matrix", which completed successfully.
• Examined the committed file contents with nl -ba doc/cpp20-compiler-support-matrix.md | sed -n '1,220p' to validate the document text and links, which displayed as expected.
• Created the PR metadata via the repository make_pr helper, which returned the PR title and body payload successfully.

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Codex Task