Updates to the Code Style Guide

[eXpl0it3r]

• All/most of the code formatting rules can be replaced with information on how to run clang-tidy
• Add reference to enforced clang-tidy rules, see also https://github.com/SFML/SFML/blob/master/.clang-tidy
• Add a policy for pass-by-value for trivially copyable classes, see also pass sf::Vector2<T> and sf::Vector3<T> by value instead of by references SFML#3047
• Add a policy for cast usage, see also Remove unnecessary static casts SFML#3061
• Add a policy about usage of auto, especially regarding type inference, see also Simplify sf::Sprite implementation and reduce branches SFML#3067 (comment)
• Add a policy regarding usage of struct vs class, see also Use struct when all type members are public SFML#3165
• Add a policy regarding example code writing:
  • Define event model should be promoted by default
  • Define error handling when loading resources
• #pragma once, see also Change include guards to #pragma once #258