Update quickstart to make it more streamline

[abravalheri]

Summary of changes

• Rework installation to actually reflect the way we expect people to use setuptools.
• Remove references to the history of Python packaging... Beginners don't necessarily know how things used to be and adding this type of content may confuse them.
• Restructure remaining sections as part of "Overview".
• Change Package discovery section to better reflect the new auto-discovery mechanism.

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[abravalheri]

Hi @codeandfire, I am starting to work in this document to make it more clear for beginners...

It is still not meant to be a fully fledged tutorial (I am afraid those will come later).

Please feel free to give any pointers if you have the time 😄

[codeandfire]

Hi @abravalheri, I think the changes that you have made to this page, especially the way you have restructured it, are really great! The page does look much more approachable for beginners. I think you should really go ahead with this!