docs/more concise version of custom imports using NODE_OPTIONS

[thescientist13]

Related Issue

N / A

Summary of Changes

1. Provide alternative variant of using --import for custom imports, using NODE_OPTIONS
2. Updated website build script to use this flavor

# before
$ node --import @greenwood/cli/register ./node_modules/@greenwood/cli/src/index.js build

# after
$ NODE_OPTIONS='--import @greenwood/cli/register' greenwood build

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