Add example of using a CTE

[ianks]

This PR adds an example of using a CTE. I think examples like this are good to have because it illustrates how powerful ROM can be in advanced scenarios.

This example might be more fitting under some Advanced section, but currently, there is not one. Do you think tit would be worthwhile to do that or should I save that for a later PR?

[solnic]

This is very helpful, a couple of remarks though:

• Reaching out to Sequel directly is possible but discouraged, because if something is not possible in rom-sql then it's a missing feature, and we gotta add it; however, it shouldn't of course block anybody from doing advanced stuff. It would be good to somehow explain this
• It feels like this kind of stuff should be moved to "Advanced" section of SQL docs

WDYT?

[ianks]

Agree with advanced section and explanation that it is an escape hatch. I'll update.