docs: add slidev-addon-stem to the list of addons

[yutaka-shoji]

Please add my addon slidev-addon-stem to Addon Gallery | Slidev.

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@slidev/client

npm i https://pkg.pr.new/slidevjs/slidev/@slidev/client@2065

create-slidev

npm i https://pkg.pr.new/slidevjs/slidev/create-slidev@2065

create-slidev-theme

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@slidev/parser

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@slidev/cli

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@slidev/types

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commit: 7570012

[kermanx]

Thanks~