Add info about optional chaining

[Pistus]

Added basic info about the optional chaining operator linking to MDN for more info. It will now be searchable. But the placement in the operator hierarchy wasn't obvious. It can be under Logical operators like nullish coalescing or as it's own section like in MDN.

🤔: As logical operator ⭐️: As its own operator type 💬: \

[Bohooslav]

I think it should be added to logical operators.