Sharding: Clarify conflicting shard-per-CPU guidance in sharding documentation

## Description

The sharding performance guide contains conflicting guidance regarding the shard-per-CPU ratio that needs to be reconciled:

**Line 63** (in "Shard size vs. number of shards" section):
> In large clusters, this often means fewer shards than total CPU cores, as larger shards can still be processed efficiently by multiple CPU cores during query execution.

**Lines 137-140** (in "Avoid under-allocation" section):
> To increase the chances that a query can be parallelized and distributed maximally, there should be at least as many shards for a table than there are CPUs in the cluster.

These statements conflict and leave operators without a clear rule.

## Proposed Solution

Reconcile the sections by:
- Distinguishing the "starting point" baseline (aim for #shards >= #CPUs per table) from scenarios where running fewer shards is acceptable
- Clarifying when large clusters with larger shard sizes (5-50 GB) can use fewer shards than total CPU cores
- Adding bridging text that explains the conditions and trade-offs that justify running fewer, larger shards
- Ensuring both paragraphs tell a consistent story

## References

- PR: https://github.com/crate/cratedb-guide/pull/355
- Comment: https://github.com/crate/cratedb-guide/pull/355#discussion_r2410528221
- File: `docs/performance/sharding.md`
- Requested by: @amotl