doc: fix sorting in buffer.md

This PR places
`Buffer.from(object...)` section before
`Buffer.from(string...)` section
and upper-cased bottom references before backtick-started ones.

PR-URL: https://github.com/nodejs/node/pull/25477


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>