docs: add local environment instruction in Quick Start

[zungeru]

Hello,

I added steps to run the "npm install" and "npm run dev" commands in the "Quick Start" section because it may not be immediately clear to folks how to start the development server.

Summary

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [x] Docs
• [ ] Build-related changes
• [ ] Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• [x] Chrome
• [ ] Firefox
• [ ] Safari
• [ ] Edge
• [ ] IE

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature
• [ ] Related documents have been updated
• [ ] Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[GregoireF]

Hi @zungeru,

Thank you for your interest in VuePress 1.X :heart:

I think this request can be closed due to an obvious evolution of VuePress 1.X

Here is an example in guide/getting-started :

6. Serve the documentation site in the local server

   yarn docs:dev
   

   npm run docs:dev
   

   VuePress will start a hot-reloading development server at http://localhost:8080.