Explain how "macros" work in djot

[matklad]

It's not obvious that djot is, in fact, an extensible format. People are looking for macros, or syntax extension, not find them, and think that it's basically markdown situation.

I think a nice way to improve this would be to have a "Syntax Extensions" section in the docs, which basically says that djot doesn't really need those.

[jgm]

This is a good idea, though I might want to think more about the content.

Note also: the section on Filters in https://github.com/jgm/djot.js/blob/main/doc/djot.md