Document how to customize \d+ to report distributed table sizes

## Why are we implementing it? (sales eng)

### What are the typical use cases?

People using psql want to use a shortcut to find table size. In psql this shortcut is `\d+`, which calls `pg_catalog.pg_table_size`. Currently that size will under-report for distributed tables. We have a custom Citus function people can call for the true size, but it would be easier to fix the default shortcut.

## Communication goals (e.g. detailed howto vs orientation)

Give people the sql snippet they can use to fix `\d+`.

### Good locations for content in docs structure

https://docs.citusdata.com/en/stable/admin_guide/table_management.html#determining-table-and-relation-size

## How does this work? (devs)

### Example sql

https://gist.github.com/marcocitus/cc3cdd528e4d2e8e35b5e75eeca986c6

### Corner cases, gotchas

https://gist.github.com/marcocitus/cc3cdd528e4d2e8e35b5e75eeca986c6

At time of linking, that gist does not handle errors. It should catch errors to be sure that orig_table_size gets called in the worst case. Alternately, perhaps Citus could run the snippet during install so nobody would have to think about it?