Add an introduction section which explains the terms “major version”,
“minor version”, and “patch level”, explains the difference between
updating and upgrading, and outlines what sort of upgrades we do and do
not support.
It is unfortunate that the FreeBSD support model is not documented in
the Handbook or, indeed, anywhere except our mailing list archive. As a
compromise, this introducion links to the security page on the website,
which briefly mentions the support model and links to the relevant
emails in the archive.