Write tutorials on tasks a user might want to do

[vringar]

We want to make it easy for users to customize OpenWPM to their needs, so we should provide guidance on how to do it most effectively. All of these tutorials should be under docs/tutorials

• [ ] Write a tutorial on how to create a new command
• [ ] Write a tutorial on how to create a WebExtension instrument in JS
• [ ] Write a tutorial on how to create a WebExtension instrument in TS