docs(configuration): add information about mozaic.config.js

[AdeoRemiDoolaeghe]

I have read the contributing guidelines

• [x] Yes
• [ ] No

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Describe the changes

Explains how to define the Mozaic configuration file, depending on your module system (ESM/CommonJS)

GitHub issue number or Jira issue URL: N/A

Other information

I'm clearly open to changes, as I had to make choices on my own that could not fit very well with the Mozaic way. Don't hesiate to propose changes