Add NPM as a requirement for the installation of Mautic

[RCheesley]

This needs to be in our end-user docs and also at mautic.org/download/requirements - we now require NPM because it's used for building JS dependencies but people are coming up against problems because it's not clearly documented.

Forum threads:

https://forum.mautic.org/t/unable-to-install-mautic-5-0-2-with-composer/30670

https://forum.mautic.org/t/composer-setup-for-mautic-5-now-needs-npm/29823

https://forum.mautic.org/t/mautictwigtemplatesbundle-not-shown/30595/6

1. Perhaps we can document the specific extensions needed for installing Mautic (@jos0405 do you have this documented already somewhere?)
2. We should include this in the composer docs updates, also.