Add reference docs for `mc du`

[ravindk89]

Summary

We should add reference for the mc du command. We often reference it for support cases where users want to know the immediate real data usage and distribution of a bucket or prefix, vs waiting for the Scanner to walk those paths.

Goals

• Add the mc du reference page to docs
• Update the mc reference index page to include mc du

Non-Goals

Anything else on usage outside of a reference context

[feorlen]

Lost the final commit due to network outage. It's in #789 instead.