intro(9): rewrite from scratch

This page has existed as a placeholder since its creation in 1995. It
does not provide a useful introduction to the content in this section.

Reimagine it as a top-level overview page containing brief descriptions
and links to existing pages in section 9. It is roughly organized into
sub-sections, grouped by topic or subsystem. In other words, the page is
meant to function as a map to other content.

There is a balance to be found here between providing as many links as
possible and keeping the page concise and searchable. In general the aim
is to reference pages which provide the best entry point to a particular
topic. For example, a link is given to locking(9), but not to the
specific lock pages such as mutex(9) or rwlock(9).

NetBSD has done something similar with their intro(9), so some
inspiration has been taken from there, although their content doesn't
align that closely with what we have.

I have done a thorough review of our existing pages and formed these
subsections around them, but they are meant to evolve.

PR: 270481
Reviewed by: imp, emaste
MFC after: 3 weeks
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41104

(cherry picked from commit 84f9f2c5cf7841fffc03ccb1833814892ae15132)