Add basic diagram with modules of coturn inside Developer.md.
I also wanted to move the existing Flow chart to mermaid so that our doc
doesn't depend on external tools but i couldn't find the time to do it
properly (horizontal arrows are hard to represent with mermaid) so i
postponed that part.
This is the result of the mermaid diagram:

The flowchart will be kept updated:
- Use [drawio](https://app.diagrams.net/) to edit.
1. Open [drawio](https://app.diagrams.net/) in brower
2. Menu → File → Open from ... → Device:
Select [FlowChart.html](FlowChart.html)
3. Edit flow chart
4. Export to svg:
Menu → File → Export as... → SVG...