Document citus_columnar as a separate extension

With #5911, we enabled `citus_columnar` to be installed/created as a separate extension:

```SQL
\dx
List of installed extensions
┌────────────────┬─────────┬────────────┬──────────────────────────────┐
│ Name │ Version │ Schema │ Description │
├────────────────┼─────────┼────────────┼──────────────────────────────┤
│ citus_columnar │ 11.3-1 │ pg_catalog │ Citus Columnar extension │
│ plpgsql │ 1.0 │ pg_catalog │ PL/pgSQL procedural language │
└────────────────┴─────────┴────────────┴──────────────────────────────┘
(2 rows)

[local] onderkalaci@onderkalaci:9700-1692=# show shared_preload_libraries ;
┌────────────────────────────────────┐
│ shared_preload_libraries │
├────────────────────────────────────┤
│ citus_columnar, pg_stat_statements │
└────────────────────────────────────┘
(1 row)
```

This is not documented in Citus readme or citus docs. We should improve it.