FAQ - Add info on disk usage

[eh-am]

As this is a common question (eg
https://pyroscope.slack.com/archives/C01FJRYENPQ/p1663579577381859?thread_ts=1663577712.177039&cid=C01FJRYENPQ) we should add it to the faq (https://pyroscope.io/docs/troubleshooting-faq/)

We could base off @petethepig's answer:

this can vary greatly depending on the app, if you’re only using it for local profiling purposes it shouldn’t ever go above 10 GBs. With production usage I’d give an estimate of 250 GB for a 1000 clients and 1 month retention.