doc: remove SmartOS from official binaries

Based on feedback from Joyent, the SmartOS builds are largely unusable
without bundling all of the libraries used to build the binary so are
being dropped starting from Node.js 14. We will still test SmartOS in
the CI.

Signed-off-by: Richard Lau <riclau@uk.ibm.com>

PR-URL: https://github.com/nodejs/node/pull/32812
Refs: https://github.com/nodejs/build/issues/2168


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>