[Document]: add `rspack-examples` to `docs/en/guide/start/index.mdx`

[chenjiahan]

This is an issue created for first-time contributors, PR welcome! 😄

Add the link of rspack-examples to docs/en/guide/start/index.mdx and docs/zh/guide/start/index.mdx, the content should be the same as:

• https://github.com/web-infra-dev/rsbuild/blob/main/README.md?plain=1#L78
• https://github.com/web-infra-dev/rsbuild/blob/main/README.zh-CN.md?plain=1#L78

[bottlewook]

I'll do this, assign it to me!

[chenjiahan]

done 😄