doc: How to plug a presenter

Context

Closes #56 (closed).

What has changed?

Main Changes

core

• Allow multiple presenters: There is no reason to prevent it.

doc

• Add explanations, similar to existing ones for solvers and dictionaries

Others

doc

• Update SPI versions: It should be done on every release.
• Add .kts extension to settings.gradle: It was forgotten by the migration to Gradle Kotlin DSL.