docs: toggle vite-plugin-llms by document language

[toreis-up]

This pull request introduces a conditional plugin loading mechanism in the Vite configuration for the documentation site. The main change ensures that the llmstxt plugin is only loaded when the root locale label includes "English", optimizing the build process for different language documents.

Plugin loading improvements:

• Added logic to check if the root locale label includes "English" by importing the config and evaluating config.locales?.root.label in docs/vite.config.ts.
• Updated the plugins array to load the llmstxt plugin only when the documentation is in English, preventing unnecessary plugin execution for other locales. (https://github.com/okineadev/vitepress-plugin-llms/blob/1b5aeadb0274836e72d63e48b8aaa0075c0905fc/README.md?plain=1#L49-L51)

ref: slidevjs/docs-ja#10

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commit: 075dddf