SEO - Add meta descriptions to each page

A description tag for each page can be added by exporting a variable named `description` from each mdx file. e.g.:

```
export const description = "This will be the description"
```

The issue points to each MDX file and shows the expected exported `description` variable. Locate the repository’s MDX pages, review how page metadata is consumed, and add a suitable description export to every page; done means each page has a description available for its meta tag.

由索引模型根据 Issue 内容生成。