feat(4243): add doc for create an analystics event from extension

[DDDDDanica]

Create a new contributor-docs explaining minimum knowledge required to create an analytics event in the MetaMask extension codebase.

[gauthierpetetin]

Thanks a lot @DDDDDanica for kicking off the tutorial on analytics in Extension!
@NicolasMassart since you have worked a lot on analytics, it would be great to have your insights as well, so I added you as a reviewer. We will also need to include content for analytics in Mobile, probably in the same PR. We can discuss this during the weekly review.