docs: avoid large Table of Contents at top of HTML version of manual

Texinfo-6.8 (that was used to build the docs of nano-5.9) changed
the way the @contents command is processed for HTML output, with as
result a detailed table of contents at the start of the HTML page.
That is ugly and too much detail.  To avoid that, produce a table
of contents only when generating output meant for printing.

This fixes https://savannah.gnu.org/bugs/?61344.

Bug existed since version 5.9.

doc/nano.texi

@@ -1990,6 +1990,8 @@ as nano's settings file, instead of the default @code{.nanorc}.
 <hr>
 @end html
 
+@iftex
 @contents
+@end iftex
 
 @bye