docs(webpack): document sandboxed preload default for Forge 7.x

[claude]

Requested by Samuel Attard · Slack thread

Fixes electron/forge#3659.

Before / After

Before: The Webpack Plugin page showed how to attach a preload script to an entry point and how to consume the *_PRELOAD_WEBPACK_ENTRY global, but said nothing about how preload scripts are compiled. A reader upgrading from Forge 6 to 7 whose preload used require or other Node.js APIs would find it silently broken with no explanation, and no guidance on how to fix it.

After: The page gains a dedicated "Preload scripts" section that tells the reader:

• Since Electron 20 (and by default in Forge 7.x), preload scripts are sandboxed — the plugin compiles them with the sandboxedPreload webpack target, so require, process, and Node core modules are unavailable, and the recommended pattern is contextBridge.
• This is a behavior change from Forge 6, called out with a warning hint for upgraders.
• How to opt back into full Node.js access when needed: either set nodeIntegration: true on the window entry point (switching the preload to the electronPreload target), or declare a standalone preload-only entry point with its own nodeIntegration: true.
• How the preload global is named for a preload-only entry point (<NAME>_PRELOAD_WEBPACK_ENTRY), with a worked example matching what users hit in the issue.

How

I confirmed current 7.x behavior against packages/plugin/webpack in electron/forge: WebpackConfig.ts derives the preload target as (entry.nodeIntegration ?? rendererOptions.nodeIntegration) ? 'electronPreload' : 'sandboxedPreload', and Config.ts defines nodeIntegration on the base entry-point interface (not on the preload object) plus a WebpackPluginEntryPointPreloadOnly type. The new section documents exactly this. Docs-only change to config/plugins/webpack.md; electron-markdownlint and lint-roller-markdown-standard both pass locally.

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