doc: edit BUILDING.md

I should have done this a few days ago so it could land in time for Code
& Learn, but oh well. Here are some revisions to BUILDING.md to try to
make it easier to read.

PR-URL: https://github.com/nodejs/node/pull/23435


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>