Overhaul open source page

[Kaptard]

The current open source page is just using a very basic list layout. However, since the open source approach is integral to our project, we should do as good of a job explaining the concept as possible.

The core message of the page is supposed to be that we're an open community that welcomes anyone who wants to shape the future of this project.

Sub goals of the page include:

• Full transparency for our users. Be it for trust or to serve their curiosity about the project.
• Explain the soon to be implemented bounty system
• Adding features is easy. Link to some basic examples that explain how it works for API endpoints and UI additions. (this should be the second-most important part next to the community aspect. Would make a good CTA.
• At the end of each example, link to the 'Add a new Feature' page or our issue page if people wanna fix existing problems.

Some useful references:
https://opensource.google.com/