Add an UI extension exemple that display proxy cache hit/miss

[obriat]

Prerequisites

• I am using the latest version of Locust
• I am suggesting a new feature, not asking a question

Description

I just write a UI extension that count my CDN hit/miss per path.
@cyberw suggests on Slack that I should add this exemple to the code, so here I am 🙂.
I will push this as a PR soon, I will also add some details to the extending-web-ui documentation base on my experience: migration to new ui, add deprecation on older examples, tips, ...