Document the requirement/recommendation to use Yarn 4.x over 1.x

[arinoch]

Pitch

Mastodon's installation instructions still suggest using Yarn 1.x (yarn set-version classic) during the install process. Given the direction we're going, Glitch-soc's intro doc should advise people to upgrade Yarn (yarn set-version stable).

Motivation

Upgrading Yarn resolves an issue that only affects the glitch install of mastodon. Vanilla Mastodon is, at this time, not affected. Since we recommend following Mastodon's installation steps to get started, adding this as part of the additional steps to follow when migrating to Glitch is strongly suggested.