docs: change installation documentation for more consistent package manager usage.

[joezimjs]

Description

The documentation showed npm being used to install the tool, but then yarn to run a server. This changes the examples to all use npm for consistency's sake. Alternatively, you could switch all code blocks to yarn, or add example code for each package manager.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).