Write a comprehensive page on the process of making an MRP

**Your Godot version:**

N/A

**Issue description:**

Making an MRP is a complicated and difficult process even for experienced contributors, and after discussion in rocket-chat and general experience with triaging I believe we should create a page with instructions on how to effectively make a good MRP, with things like:
* Tips on how to identify the minimal state if you are starting from an existing project
* How to create a good MRP from scratch with ideas for how to showcase the behaviour effectively, such as:
- ideas for adding interactivity to trigger behaviour
- ideas for adding configurability to make it easier to display different cases
- good sources for free and permissively licensed material to put into your project, such as audio, art, meshes, etc., this has often been a struggle for me working with making MRPs
* Instructions to make sure that the files you include are yours to share, and that you check the licensing if you have, for example, purchased assets for your project, so that people don't accidentally violate their asset licenses when including them in an MRP

Further ideas are welcome on what to put into this

**URL to the documentation page (if already existing):**