Fix *roff string usage nit

String definitions were being used with a pointless leading double
quote.  This syntax is used only to define strings containing leading
space characters.  (You might also use it defensively if you're defining
one string whose contents start with the interpolation of another, and
the latter might interpolate leading space--but that is not the case
here.)

Remove unnecessary leading quotes from string definitions.

* doc/Guidebook.mn: Do it.  Also annotate empty strings with comment.
* sys/unix/hints/include/gbdates-post.370: Don't put them back.

https://www.gnu.org/software/groff/manual/groff.html.node/Strings.html

doc/Guidebook.mn

@@ -33,14 +33,14 @@
 .if n .do ftr CR R  \" if formatting for a terminal, substitute R for CR
 .                   \" (the substitution affects both '\f' and '.ft')
 .
-.ds h0 "NetHack Guidebook
-.ds h1
+.ds h0 NetHack Guidebook
+.ds h1 \" empty
 .ds h2 %
-.ds vr "NetHack 3.7.0
-.ds f0 "\*(vr
-.ds f1
-.\"DO NOT REMOVE NH_DATESUB .ds f2 "DATE(%B %-d, %Y)
-.ds f2 "November 6, 2023
+.ds vr NetHack 3.7.0
+.ds f0 \*(vr
+.ds f1 \" empty
+.\"DO NOT REMOVE NH_DATESUB .ds f2 DATE(%B %-d, %Y)
+.ds f2 November 6, 2023
 .
 .\" A note on some special characters:
 .\" \(lq = left double quote