Update description/title of Partition Balance visualization

[hcoyote]

Discussion here: https://redpandadata.slack.com/archives/C02JDTMG8GN/p1703184448433599

tl;dr: Today we have a "Partition Balance" graph that provides a percentage-based view of how balanced the partitions are on the cluster.

The intention of this graph is to show a red/yellow/green view of how balanced the partition writes are, not how balanced the partitions are across the cluster (e.g., the spread of partition counts on each broker).

We should update the title and add a description to clarify this.

If we don't already have it, we should probably also have a separate metric pointing to the partition count spread.

[hcoyote]

We should probably get this fixed sooner rather than later. Another thread where someone found it confusing.

https://redpandacommunity.slack.com/archives/C01AJDUT88N/p1705983556876499