Lay the guide out as Common, Cpp17 and Cpp20 with an index

The guide moves to Progmasoft_Core_Cpp_Coding_Guidelines/:

- Common/ holds the thirteen chapters, which apply to C++17 and C++20
  alike. Where a chapter used a C++20 feature, it now states the rule
  for both versions and links to the page of each.
- Cpp20/LANGUAGE.md holds the rules that use C++20 features: building
  with -std=c++20, designated initializers, consteval and constinit,
  std::bit_cast, concepts, defaulted comparisons, ranges, std::span,
  std::format, the smaller features, and the C++20 features not used.
- Cpp17/LANGUAGE.md holds what a C++17 project does instead: building
  with -std=c++17, the C++17 form of each C++20 rule (the former
  section 5.13) and the checks a C++17 library needs for later
  standards.
- index.json lists every document with its chapter, its path and the
  versions it applies to, and names the default version.

No rule changed. The README lists the version pages and the layout,
ADOPTING.md points to the page of the chosen version, and the changelog
says where Guide/ went. Every relative link and heading anchor was
checked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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