Suggestion to add size_t reference page

[per1234]

*Moved from arduino/Arduino#6921 by @sterretje

As title says

Add a data type size_t to the data type section; maybe add a hint about sizeof to determine what the size of size_t is so people can determine for themselves.

[per1234]

I agree that it should be documented. Although I don't think size_t is a type that will be commonly used by beginners in their code, some Arduino functions return size_t, which should be documented, and that will naturally lead users to look for a size_t page in the Data Types section of the Language Reference.

[q2dg]

Well, datatypes documentation is a lost battle...see arduino/Arduino#4525