Documentation on Compile Time Evaluation is out of date

[nbonfils]

The doc for compile time function is out of date: https://c3-lang.org/generic-programming/compiletime/#compile-time-functions

I was trying to play around with macros I was tring to build an array at compile time, the only hint I found in the doc was $append which has been removed (I got the hint from the commit that deprecated it).

Then I realized that there are compile time arrays, so it could be worth to add them to the doc as well. I also am not sure what the var keyword does exactly inside macros, adding a sentence or two on that could be welcome too.

Hope this kind of report helps, really liking the language so far, thanks!