[Docs] Add favicon

[olivernybroe]

In our documentation we do not show our icon which is a big shame now that @caneco did such a nice job.

You can find the icon to use for favicon under art/heart.svg.
This has to be added in vuepress. In their docs they have a simple example of how to do it here.

[nunomaduro]

For newcomers - here is the source code: https://github.com/nunomaduro/phpinsights/tree/master/docs.

Setup locally:

git clone https://github.com/nunomaduro/phpinsights
cd phpinsights
yarn
yarn run vuepress dev