Documentation: Capacity Planning - states nonexistant metric 'prometheus_tsdb_head_series' #7325

Describe the bug
The Documentation under https://cortexmetrics.io/docs/guides/capacity-planning/ states a metric 'prometheus_tsdb_head_series' that does not exist at all!

To Reproduce
Try to find prometheus_tsdb_head_series in /metrics on your cortex.

Expected behavior
It exists on /metrics

Environment:
Docker in single-binary mode

Additional Context
There is a closed PR that tried to add this metric to the system, but the PR was never merged: #4308

There is a comment on this PR, that states, an alternative to the given metric: #4308 (comment)

So maybe the Documentation can get fixed to that?

Instead of mentioning 'prometheus_tsdb_head_series' it could say 'cortex_ingester_memory_series_created_total - cortex_ingester_memory_series_removed_total' - if this query is still correct?