Reorganize/condense intro

[katrinabrock]

If this pull request addresses an open issue on the repository, please add 'Closes #NN' below, where NN is the issue number.

#1511

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

• Cut down intro by:
  • Removing the explanation of bash acronym
  • Removing analogy to language learning
  • Providing fewer example use-cases
• Reorganized for (hopefully) better flow by:
  • Separating content into (short) "What" and "Why" sections instead of "Background" and "Shell" sections which have less clear boundaries.

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[github-actions]

🆗 Pre-flight checks passed 😃

This pull request has been checked and contains no modified workflow files, spoofing, or invalid commits.

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[katrinabrock]

@bkmgit committed your suggestions. Any other feedback or is this good to go?