docs: add Deno to install instructions

[bartlomieju]

👋 Bartek from the Deno team here.

The install instructions list yarn and npm, but not Deno. Since Deno is a drop-in replacement for npm these days, I'd like to add a Deno line so Deno users have a copy-pasteable command, in parity with the others:

deno install @emotion/react

(As of Deno 2.8 there's no npm: prefix needed, it's the same command shape as npm install @emotion/react.)

Totally fine to close this if you'd rather keep the list short, no hard feelings. Happy to adjust wording, placement, or move it elsewhere.

Disclosure: I work on Deno, so I have an interest here. The goal is parity with the package managers you already list, not promotion. This PR was prepared with AI assistance under human supervision: I review every change myself and personally respond to any comments or review feedback.

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What: Adds a deno install @emotion/react line to the install docs, next to the existing yarn and npm commands.

Why: Deno users currently have to translate the npm command themselves; this gives parity with the package managers already listed.

How: Added one bash snippet in docs/install.mdx matching the existing style.

Checklist:

• Documentation
• Tests — N/A
• Code complete
• Changeset — N/A (docs only, no package release)

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[Andarist]

To be fair, with the plethora of package managers these days - I kinda wonder if we just shouldn't mention a single package manager in those instructions. Like, Deno users know how to install packages, right? I would just like to avoid having a list of 5+ installation instructions here where each one of them is super-super similar and already well-known by the users of those tools

[bartlomieju]

@Andarist that's fine for me, feel free to close the PR!