Add an example based on the material-ui nextjs example

[janus-reith]

I would like to see an example how to integrate tss-react based on this: https://github.com/mui-org/material-ui/tree/next/examples/nextjs-with-typescript

The current way things work is a bit confusing to me:
The example in the MUI docs has a shared util createEmotionCache which when called, always returns the result of emotions createCache function.

Now both _app and _document call that, _document uses it once in initial props, _app stores it global to the file.

The example here acts in a different way: The createDocument function returns getInitialProps logic, which internally uses getCache.
There is no _app, but I assume looking at what index.tsx does is the same what _app would do per page. index.tsx also uses the same getCache function.
What getCache basically does, is it stores a cache globally, and returns in or if it doesn't exist yet, create it using createCache from emotion.
That seems to be different from what the mui example does, since _document and _app receieve their own instance here.

Some more background to why this example would help understanding the cache utilization is in #19.

Having an example using the same structure as the MUI one would help a lot to understand the differences there.

[garronej]

https://github.com/garronej/tss-react/tree/main/src/test/ssr