Format exercises, add descriptive titles

[bencomp]

This fixes #157 and fixes #121.

Regarding the titles of exercises: I feel I could use a style guideline here, as this first attempt has inconsistent forms of verbs, or no verbs at all in the title. What form is preferred? Some examples:

• "Remove a trailing space"
• "Commonly owned items"
• "Finding (potential) errors"

To fix #121 I wrapped the mentioned steps in a challenge fenced div. I did the same for some other "exercise-like walk-throughs". But now in some places there isn't much explanation left. This causes new issues like #158.

[github-actions]

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Rendered Changes

🔍 Inspect the changes: https://github.com/datacarpentry/openrefine-socialsci/compare/md-outputs..md-outputs-PR-159

The following changes were observed in the rendered markdown documents:

 02-working-with-openrefine.md | 36 +++++++++++++++++++++++-------------
 03-filter-sort.md             | 34 +++++++++++++++-------------------
 04-numbers.md                 |  9 +++------
 05-scripts.md                 | 13 +++++++++++--
 06-saving.md                  |  6 +++++-
 07-resources.md               |  4 +---
 md5sum.txt                    | 12 ++++++------
 7 files changed, 64 insertions(+), 50 deletions(-)

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⏱️ Updated at 2023-05-30 17:07:49 +0000

[bencomp]

I just noticed that the explanation of clustering is not yet made into an exercise.