docs: enforce rune mode to universal reactivity tutorial

[sacrosanctic]

closes #1229.

I've gone through all the similiar reported issues and there is only 1 tutorial with this problem.

Disabling for just this tutorial should resolve most users issues.

Todo

• an explanation can be added

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[Rich-Harris]

good catch, thank you! I think that we should opt all the tutorial content into runes mode, rather than individual components — opened #1273

[Rich-Harris]

merged #1273 — closing