docs: Update making plugins.md (#567)

Anson Yu

2023-11-04 aac7b7e97d32580f6a9bd974b8a9068e5218ab9c

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$pageWidth: 750px;
$mobileBreakpoint: 600px;
$tabletBreakpoint: 1200px;
$sidePanelWidth: 380px;
$topSpacing: 6rem;
$fullPageWidth: $pageWidth + 2 * $sidePanelWidth;