docs: add eslint to install command

[KumJungMin]

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What is the purpose of this pull request?

The @eslint/markdown package does not declare eslint as a peer dependency.
As a result, users may assume that installing @eslint/markdown will also install ESLint, but ESLint must actually be installed separately in order to use the plugin.

The existing installation instructions only include @eslint/markdown, which may cause new users to miss a required dependency. To prevent this, the documentation now instructs users to install eslint before installing @eslint/markdown.

What changes did you make? (Give an overview)

Updated the installation instructions for @eslint/markdown to clarify that eslint must be installed separately before installing the plugin, consistent with the change made in eslint/json#258.

Related Issues

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[linux-foundation-easycla]

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• ✅ login: KumJungMin / name: KumJungMin (eefbfca)

[nzakas]

LGTM. Thanks.