Canonical way to set TOC depth

Use `\placecontent` instead of `\placelist` as `\placecontent` is easier to
configure.

default.context

@@ -45,6 +45,10 @@ $endif$
    indentnext=no,
   ]
 
+$if(toc)$
+\setupcombinedlist[content][list={$placelist$}]
+$endif$
+
 $for(header-includes)$
 $header-includes$
 $endfor$
@@ -69,7 +73,7 @@ $for(include-before)$
 $include-before$
 $endfor$
 $if(toc)$
-\placelist[$placelist$]
+\placecontent
 $endif$
 
 $body$