docs(gatsby-plugin-image): Add docs for customizing default options

[kelvindecosta]

Description

Added documentation for customizing gatsby-plugin-image default options.

Documentation

You might find yourself using the same options (like placeholder, formats etc.) with most of your GatsbyImage and StaticImage instances.
You can customize the default options with gatsby-plugin-sharp.

The following configuration describes the options that can be customized along with their default values:

// In gatsby-config.js
module.exports = {
  plugins: [
    {
      resolve: `gatsby-plugin-sharp`,
      options: {
        defaults: {
          formats: [`auto`, `webp`],
          placeholder: `dominantColor`
          quality: 50
          breakpoints: [750, 1080, 1366, 1920]
          backgroundColor: `transparent`
          tracedSVGOptions: {}
          blurredOptions: {}
          jpgOptions: {}
          pngOptions: {}
          webpOptions: {}
          avifOptions: {}
        }
      }
    },
    `gatsby-transformer-sharp`,
    `gatsby-plugin-image`,
  ],
}

Related Issues

Refer to #29147

[LekoArts]

Thanks!