Add a demo repo link for index.html

Maybe it's better to add a demo repo link. When I try this code at first time, I don't know what dependencies should be added.

like this:

Here are my demos:

```
https://github.com/zhenfeng-zhu/basic_web_server
https://github.com/zhenfeng-zhu/multi_threaded_helloworld
https://github.com/zhenfeng-zhu/helloworld
```

Start by locating index.html and reviewing how links to examples or demos are presented. Use the demo repository URLs included in the issue, then verify that a newcomer can find a working demo and understand which dependencies are needed.

Written by the indexing model from the issue text.