add info about how to handle filepaths in electron

[Taschenbuch]

From my own experience in my team at work and from the discord here that is one of the biggest issues for people setting up electron for the first time and even later it can cause confusion. Especially the part that it may work in development mode (electron .) but not after being packed and run via .exe, can be very confusing and can cost hours/days to fix when you are new to that.

should be covered:

• relative paths are not working. Usage of __dirname to create absolute paths.
• how to debug it: typically you see a white browserwindow and an error in devTools console. Use asar: false to check content of app(.asar) folder to check if everything is there you expected. if yes, a path could be an issue.
• common pitfalls like when an ui framework is used it may need to be adjusted to work with absolute paths (?). example: create-react-app requires setting packagejson "homepage": ".". I think react requires usage of HashRouter instead of the normal router, too? Would be nice to have the fixes for common ui frameworks mentioned but maybe it is already enough to just give 1-2 examples for people to understand what it is about in general.
• when a bundler like webpack is used, the paths may change. Then "main" in package.json needs to be updated and paths used for browserWindow.loadURL/file() need adjusting.