docs: drop the cluttering Info-style links from the HTML manual

Especially: get rid of the "Up: (dir)" link that points nowhere.

Also, add a horizontal line before the final Table of Contents.

doc/Makefile.am

@@ -28,6 +28,6 @@ info_TEXINFOS = nano.texi
 BUILT_SOURCES += nano.html
 dist_html_DATA += nano.html
 
-AM_MAKEINFOHTMLFLAGS = --no-split
+AM_MAKEINFOHTMLFLAGS = --no-split -c HEADERS=0
 
 EXTRA_DIST = $(BUILT_SOURCES) $(info_TEXINFOS)

doc/nano.texi

@@ -1592,5 +1592,10 @@ curses libraries.
 
 @end table
 
+@html
+<hr>
+@end html
+
 @contents
+
 @bye