doc: add directions to mark a release line as lts

Moves directions for LTS release from the Release repo to Node docs.

Fixes: https://github.com/nodejs/Release/issues/530
PR-URL: https://github.com/nodejs/node/pull/31724


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>