Update text in 1.3 and multiple suggested revisions

[morskyjezek]

Please briefly summarise the changes made in the pull request, and the reason(s) for making these changes.

There are multiple suggestions here, but each is in its own commit, so if you all would prefer to make more incremental changes, please feel free to modify or reject.

Here's an overview of the suggestions:

• Added instructional text to element 1.3 (easter eggs) as suggested in PR lesson listed in conceptual pre-alpha section of website but is beta #61
• multiple typos or language updates
• a few minor styling updates (bold, italics, ordered list)
• and revisions to the text in some sections (to clarify or complete sections)

If any relevant discussions have taken place elsewhere, please provide links to these.

I haven't been a contributor to this lesson previously, but I hope you all don't mind these suggestions. I've worked mostly on the LC tidy data, shell, and some other small changes previously. I've also been a maintainer on the tidy data lesson.

These changes are mostly in response to some of the issues and requests noted as possible ways to refine and move the lesson along in the stages, as discussed in PR #61

[github-actions]

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