docs(config): document extraResource for runtime assets

[SAY-5]

Refs electron/forge#3621.

The Electron Packager config page lists packagerConfig and links to the Packager API docs, but does not show how to ship runtime assets (e.g. a tray icon, native binary, or other file referenced at runtime). Issue #3621 is exactly this case: a tray icon loads in electron-forge start but fails after packaging because the file is inside app.asar. The fix is extraResource plus process.resourcesPath, which is non-obvious without prior knowledge.

Adds a hint block under the Electron Packager config section with a minimal extraResource example and the matching tray-icon main-process snippet that resolves the path with process.resourcesPath when app.isPackaged.

Maintainer endorsement on the issue (after a contributor offered to write this doc):

Hi @vishalkishore, PRs welcome in https://github.com/electron-forge/electron-forge-docs

-- @erickzhao, electron/forge#3621 (comment)