docs: Move react-email to dev dependencies in install guides

[PCRinus]

react-email can be installed as a dev dependency, instead of a dependency that we need to ship at runtime, thus saving bundle space. Adjust the documentation for all package managers to reflect this.

This has been mentioned initially in this issue thread: #1101

Side by side comparison

[vercel]

@PCRinus is attempting to deploy a commit to the resend Team on Vercel.

A member of the Team first needs to authorize it.

[gabrielmfern]

Could you make all --save-dev flags into just the shorthand form, i.e., -D?

[changeset-bot]

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Latest commit: 05fb2c5

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[PCRinus]

@gabrielmfern Sorry for the delay, I've changed all the dev flags to their shorthand version.

I would keep the -D and -E flags separate instead of chaining them (-DE), since it seems easier to read and research those flags in case someone is not aware what they do.

Also, do we need to generate a changeset for this small change?

[gabrielmfern]

Thanks for making the changes!

Also, do we need to generate a changeset for this small change?

Nope, changeset is only meant for when changing something in a package that would require a subsequent versioning change.