Update quickstart guide to reflect user research

[nlhkabu]

Updates quickstart guide to reflect most common tasks as discovered in our "buy a feature" user research.

Preview: https://pip--9137.org.readthedocs.build/en/9137/quickstart/

[nlhkabu]

@pradyunsg or @uranusjr this content is ready for review.
If you can let me know if it is accurate I will fix linting and add a news entry. Thanks!

[pradyunsg]

Looks great to me! A few fairly minor comments.

[uranusjr]

#9131 (merged recently) also included similar instruction on installing and upgrading pip in the Installaton page. Is this duplication intentional, or should we simply redirect users to that page instead?

Current “latest” pip documentation: https://pip.pypa.io/en/latest/installing/

[pradyunsg]

@nlhkabu If you rebase this PR on the current master branch, that'll fix the CI here.

Is this duplication intentional, or should we simply redirect users to that page instead?

https://www.writethedocs.org/guide/writing/docs-principles/#arid

I think this duplication is A-OK.