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
• [X] Documentation update
• [ ] Other

Before submitting the PR, please make sure you do the following

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