Add references for the most important man7

pages worth reading to intro(7).

Submitted by:	Gordon Bergling gbergling_gmail.com
Approved by:	bcr
Differential Revision:	https://reviews.freebsd.org/D24649

share/man/man7/intro.7

@@ -28,7 +28,7 @@
 .\"     @(#)intro.7	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd June 5, 1993
+.Dd May 3, 2020
 .Dt INTRO 7
 .Os
 .Sh NAME
@@ -36,6 +36,44 @@
 .Nd miscellaneous information pages
 .Sh DESCRIPTION
 This section contains miscellaneous documentation.
+.Bl -tag -width "mdoc.samples(7)" -offset indent
+.It Xr ascii 7
+map of ASCII character set
+.It Xr c 7
+the C programming language
+.It Xr environ 7
+user environment
+.It Xr firewall 7
+simple firewalls under
+.Fx
+.It Xr hier 7
+file system hierarchy in
+.Fx
+.It Xr hostname 7
+host name resolution description
+.It Xr release 7
+layout of
+.Fx
+releases and snapshots
+.It Xr ports 7
+introduction to the ports infrastructure of
+.Fx
+.It Xr security 7
+security features available in
+.Fx
+.It Xr tuning 7
+general advice on tuning
+.Fx
+.El
+.Sh SEE ALSO
+.Xr man 1 ,
+.Xr intro 2 ,
+.Xr intro 3 ,
+.Xr intro 4 ,
+.Xr intro 5 ,
+.Xr intro 6 ,
+.Xr intro 8 ,
+.Xr intro 9
 .Sh HISTORY
 The
 .Nm