doc: make C++ core guidelines a reference

As our C++ codebase consists of many code that interface with third-party libraries that do not follow the C++ core guidelines, it may be difficult to enforce the guidelines without sacrificing consistency or introducing cruft that juggles with data structures.

This patch makes the C++ core guidelines a reference instead of a general guideline to follow to reduce nits that are open to interpretation during code reviews, and highlights the automatic tooling for C++ styles. We may revisit the general priority when there are better automatic tools in place.