Ultimate guide to attributes through C++23

[lefticus]

https://en.cppreference.com/w/cpp/language/attributes

• Make sure to mention it is optional what a compiler does with them
• mention which ones can/must be ignored if unrecognized
• mention how to use compiler specific attributes

[lefticus]

https://compiler-explorer.com/z/qMzxaxzc4

[lefticus]

Coming in Ep 423