docs(cli): add command descriptions

[bluwy]

Description

Closes #6940

Add descriptions to CLI commands

Additional context

With vite --help:

Before:

Commands:
  [root]           
  build [root]     
  optimize [root]  
  preview [root]  

After:

Commands:
  [root]           start dev server
  build [root]     build for production
  optimize [root]  pre-bundle dependencies
  preview [root]   locally preview production build

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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

• 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).
• Ideally, include relevant tests that fail without this PR but pass with it.