Add missing DDT documentation files and links.

.INFO.; DDT :SEND is a help text for that command.

.INFO.; DDT DOC and DDT ORDER are expected to be links.

doc/_info_/ddt.:send

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+Instructions for use of the DDT :SEND command.
+
+Usage:  :SEND <person> <Message>^C or :SEND <person>@<its> <message>^C
+<its> may be one of AI, ML, MC, or DM
+
+Type your message normally.  It may contain more than one line as needed.
+The following characters have special effects:
+Control-C  -- Send the message
+Control-D  -- Abort the message
+Control-G  -- Abort the message
+Control-K  -- Retype the message, without clearing the screen first.
+Control-L  -- Retype the message, clearing the screen first.
+Control-Q  -- Quote next character
+Control-R  -- Retype current line
+Control-S  -- Stop re-typing
+Control-T  -- Exchange last two characters typed
+Control-U  -- Kill last line, saving killed info in the "Kill buffer"
+Control-W  -- Kill last word, saving killed info in the "Kill buffer"
+Control-Y  -- Get ("Yank") last section of killed info from the Kill buffer
+	      into the message buffer
+Control-Z  -- Exit the send command, saving the message text in the "Kill
+	      buffer", so the message text can be recovered in another :SEND
+	      with Control-Y
+Rubout	      Delete last character.  Does not copy into the kill buffer unless
+	      last character also copied into the kill buffer

doc/_info_/ddt.doc

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+info/ddt.>

doc/_info_/ddt.order

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+_info_/ddt.order