Add a demo repo link for index.html

[gofenix]

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 

[gofenix]

[lpil]

Sounds good, though I think it would be best to include the source in the website repo. We could also import the file directly into the HTML and test them on CI, to ensure they are correct.