docs: add path types example for publicDir config

[chenjiahan]

Summary

Add path types example for publicDir config.

The value of name can be set to a relative path or an absolute path. Relative path will be resolved relative to the project root directory.

• Example of a relative path:

export default {
  server: {
    publicDir: {
      name: '../some-public',
    },
  },
};

• Example of an absolute path:

import path from 'node:path';

export default {
  server: {
    publicDir: {
      name: path.join(__dirname, '../some-public'),
    },
  },
};

Checklist

• Tests updated.
• Documentation updated.

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