Docs: replace deprecated yarn global examples

[SSDWGG]

Summary

• replace deprecated yarn global add examples with yarn dlx for Herb packages
• update the language server Yarn heading to avoid describing it as a global install

Fixes #1312

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restore a real Yarn install command

Yarn's dlx docs describe it as installing a package in a temporary environment and running its binary, so it does not leave herb-language-server on PATH. In this global-install section, the following step and editor configs invoke herb-language-server directly; Yarn users who follow this line will get an LSP command-not-found instead of a persistent install. The same replacement appears in the other global-install snippets and should either remain a real install or change the later command to use yarn dlx ... --stdio.

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