Add a user tutorial for the new javascript plugin template

## User story
As a user I want to quickly develop new plugins in javascript. To get going quickly, an accessible tutorial will be very helpful.

## Rationale
A user tutorial will help to onboard new plugin developers, especially because it focuses on a simplified plugin template using raw javascript.

## Implementation details
- [ ] add user tutorial to if.greensoftware.foundation

## Priority
3/5

## Size
M

## What does "done" look like?
New page exists

## Deadline
tbc