Document column limitations

Columns of type `dynamic` have a default size limitation of 1MiB https://learn.microsoft.com/en-us/kusto/query/scalar-data-types/dynamic . In the case of the `operator` component, when creating heartbeat rows that contain schemas that exceed the default 1MiB limit, a `null` is inserted instead of truncating the result. If a user encounters this, they can create a `ManagementCommand` like

```yaml
apiVersion: adx-mon.azure.com/v1
kind: ManagementCommand
metadata:
name: heartbeat-schema-bigobject32
namespace: adx-mon
spec:
database: ""
body: |
.alter column .Schema policy encoding type='BigObject32'
```

that will enable the schemas to grow out to 32MiB before they hit the column limitation.

Update the `operator` documentation for the described limitation and workaround.