Add "on conflict" to RT use case

(Original issue description)

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

We often get questions about best practices for doing roll-ups. Although we have a little bit about this in the existing docs, we could give more detailed advice:

* Best practices
* Common patterns
* When to do rollups (Hourly, daily)
* How do deal with delayed data
* Upsert/Insert

### Good locations for content in docs structure

Would probably fit in the Administration section by the upcoming section about useful queries.

Note that this could also be a blog post, since it's relevant to PostgreSQL in general, not just Citus.

## How does this work? (devs)

### Example sql

### Corner cases, gotchas

### Are there relevant blog posts or outside documentation about the concept/feature?

### Link to relevant commits and regression tests if applicable