Add more information to `Translation contexts` section of `Internalizing games` page

**Your Godot version:**

4.3

**Issue description:**

The `Translation contexts` section of the `Internationalizing games` page needs more information. It currently only mentions the value of having contexts, and demonstrates the consuming API, but it says nothing about how these contexts are actually added to the translation assets that are passed to translators.

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/stable/tutorials/i18n/internationalizing_games.html#translation-contexts