docs: Add course content for Day 5

[holyMolyTolli]

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[kacperlukawski]

I left some comments regarding the lessons, and here are some important ones:

1. Filtering does not work as described. Global filters are propagated all the way to prefetches.
2. The Colbert reranking does not seem to be used correctly. FusionQuery does not accept a query parameter, so even if the Python client does not complain, it is not used. We probably need to nest one prefetch operation more.
3. Recall is often used when we speak about retrieval. The latter one is more commonly used, so I tried to replace it everywhere.

In general, I would prefer to remove some parts that are mostly about filtering. We covered that already, and it's making the lessons longer than they could be.