Update docs to reflect trace combinator.
The REPL no longer prints an evaluation trace by default but the docs still show that.
Also the trace combinator is only really mentioned on the docs for the `joy.utils.pretty_print` module, it should be mentioned in the library docs and on the main introduction page.
sforman sforman 
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The REPL no longer prints an evaluation trace by default but the docs still show that.
Also the trace combinator is only really mentioned on the docs for the `joy.utils.pretty_print` module, it should be mentioned in the library docs and on the main introduction page.