Add better information about time series and how to avoid generating too many

[na--]

With k6 v0.41.0, some scripts that previously worked fine with millions of different time series will no longer be OK (grafana/k6#2765).

We should add more warnings about that in the Metrics and Tags and Groups sections. Maybe polish a little bit the URL grouping section and link it from more places, or move it to its own page? 🤔

[na--]

This is probably one of the more important things we should prioritize, since a lot of users keep hitting this issue 😞 See grafana/k6#2762, grafana/k6#2836, grafana/k6#2876 (though I don't have a good idea if people on the forum also reported it).

https://k6.io/docs/using-k6/http-requests/#url-grouping should definitely be more prominent and better explained as well. I just noticed that http.url is not even properly linked from that page 😞

[ppcano]

Related to #861