feat: add a guide on materialized ctes to query-data

[rdmolony]

Description

I just found out about materialized CTEs and thought I really should share this for people in similar situations.

I'm consulting with a company that hides TimescaleDB behind a task queue, & some jobs were taking ages. Using with cache as materialized ( ... ) resulted in a query that was taking days now takes 30 seconds! These last few months I was getting frustrated that the Postgres query planner kept deciding on NestedLoop joins.

I see that you guys cover this in https://www.timescale.com/learn/how-to-use-common-table-expression-sql - however I reckon it could be covered in the query-data section of the docs too to force this on people?

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Fixes #[insert issue link, if any]

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[jonatas]

Amazing @rdmolony! @billy-the-fish will love it!

[billy-the-fish]

I love it indeed. @atovpeko can you review please.