Add theme option to set the search bar placeholder message

[eugelito]

I would like to request an ability to add a way to set my own placeholder message in the search bar. I was thinking this could be a theme option.

Theme Options

Name	Type	Default	Description
searchPlaceholder	string	"Search"	Provide a custom placeholder message to the search bar.

// In your `docusaurus.config.js`:
module.exports = {
  // ... Your other configurations.
  themes: [
    // ... Your other themes.
    [
      require.resolve("@easyops-cn/docusaurus-search-local"),
      /** @type {import("@easyops-cn/docusaurus-search-local").PluginOptions} */
      ({
        // ... Your options.
        // Set custom message for search bar placeholder
        searchPlaceholder: "Search documentation",
      }),
    ],
  ],
};```