Cover C++17 as well as C++20docs/cpp17-and-cpp20

A project now picks one version. C++20 stays the default, and C++17 is
for libraries meant for other people's projects, because it is still the
widest common target and works in every later standard.

Section 5.13 gives the C++17 form of each C++20 feature the guide uses
and the checks such a project needs: CI in the versions it promises, with
each compiler, and headers that compile in default compiler modes.

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

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