Add a legend to version table

**Context**

In the [list of the Unity Docker images](https://game.ci/docs/docker/versions) there are little icons that show the status of the container. These are not explained anywhere and one must read the [code](https://github.com/game-ci/documentation/blob/71d71182efa22e06d7bc59b9441be5df213ba38c/src/components/docs/versions/unity-version.tsx#L29-L35) to get some understanding.

**Suggested solution**

Add a legend with each emoji and what it maps to at the top of the page.

Additionally, add a tooltip to the legend explaining what the state _actually_ means. While "Completed" is self explanatory, what does "superseded" actually mean, and do "failed" jobs get reattempted? These things are unclear.

**Considered alternatives**

Put a tooltip on the icon instead explaining the status

**Additional details**

Im very confused if my editor version is going to be created or not.
Side note, just a personal preference but I think `❌` is a better emoji for failed.