[Windows, Mac] Improve developer documentation

[ItsSim]

The Developer Documentation (https://github.com/ItsSim/fsolauncher/wiki/Development-guide) should be expanded to include the following topics:

• Contribution rules (Basic rules for pull-requests).
• Code style (dictated by ESLint rules, but just a more formal description).
• Using IPC (via library/ipc-bridge.js) to communicate between the renderer and main processes.
• Using modals (via library/modal.js).
• Creating new launcher tabs.
• Adding new components to the Installer tab.
• Debugging (main and renderer).

[tomgrice]

This would actually be very useful haha. I've actually learnt quite a bit about node and electron from trying to fix one thing. I haven't actually used electron or node before and haven't used JS in some time, so the extra guidance on code style etc would be helpful.

[ItsSim]

Added two pages to the developer documentation:

• Code Style
• Using IPC

[ItsSim]

Added two more pages:

• Using Modals
• Creating Sidebar Tabs and Pages

[ItsSim]

Added all planned documentation to the developer documentation.

• Creating Installers
• How to Contribute (CONTRIBUTING.md)
• Debugging