Add optional exercises for section 1 and 2

[svenvanderburg]

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Fixes #258 for section 1 and 2.

This adds a new episode at the end of the section with optional exercises. We could also choose to have 1 episode as extra material with optional exercises for all episodes.

Current status: section 1 and 2 done. This can be reviewed now or wait for all sections to have optional exercises, I think it would be nice to review now to make sure this is in line with thoughts the other contributors