intro.7: Add missing manual page

Section 7 of the manual pages contain lots of very useful information, but
finding the pages is not always obvious - to assist people in finding the
information, add missing cross-references.

Reviewed by:	0mp (mentor), mhorne, yuripv
Approved by:	0mp (mentor
Differential Revision:	https://reviews.freebsd.org/D27284

share/man/man7/intro.7

@@ -28,7 +28,7 @@
 .\"     @(#)intro.7	8.1 (Berkeley) 6/5/93
 .\" $FreeBSD$
 .\"
-.Dd May 3, 2020
+.Dd November 19, 2020
 .Dt INTRO 7
 .Os
 .Sh NAME
@@ -37,12 +37,29 @@
 .Sh DESCRIPTION
 This section contains miscellaneous documentation.
 .Bl -tag -width "mdoc.samples(7)" -offset indent
+.It Xr arch 7
+supported CPU architectures and platforms
 .It Xr ascii 7
 map of ASCII character set
+.It Xr build 7
+build instructions for
+.Fx
 .It Xr c 7
 the C programming language
+.It Xr clocks 7
+system timekeeping clocks available in
+.Fx
+.It Xr crypto 7
+cryptographic algorithms provided by OpenCrypto in
+.Fx
+.It Xr development 7
+development introduction to
+.Fx
 .It Xr environ 7
 user environment
+.It Xr ffs 7
+commonly used filesystem in
+.Fx
 .It Xr firewall 7
 simple firewalls under
 .Fx
@@ -61,6 +78,13 @@ introduction to the ports infrastructure of
 .It Xr security 7
 security features available in
 .Fx
+.It Xr stats 7
+statistics utilities available in
+.Fx
+.It Xr tests 7
+introduction to the
+.Fx
+Test Suite
 .It Xr tuning 7
 general advice on tuning
 .Fx