Improve onboarding: Add more details to setup tutorial

[alexabruck]

As a first time user, I got stuck in the "Setup" part.
It would be nice to have a step-by-step tutorial on how to set up Frametrail locally with MAMP.

This could include:

• frequent pitfalls, e.g. forgetting the port number
• customization tutorials, e.g. how to increase the accepted media upload size in MAMP & MAMP Pro

Nice to have: A small explainer why it is necessary to set up a server anyway. Some users might be used to a backend that is entirely managed by the company.

[OpenHypervideo]

Good points! I'll improve the tutorial and the error messages in FrameTrail accordingly.