Add OSS to the quickstart

The quickstart is cloud-centric. Because OSS is a popular path for first-time users, let's update the quickstart to give people the option of whether they want to go down the cloud or OSS path.

Closes https://github.com/PrefectHQ/prefect/issues/14029

This PR also converts the quickstart to ventilated prose to make future edits easier to review.

Example

Instead of creating separate quickstarts for cloud and OSS, let's add tabs to the existing quickstart to let people easily toggle between these paths. This approach doesn't require users to know in advance which path they want to go down, and it makes it easy to compare equivalent cloud and OSS code.

<Tabs>
  <Tab title="Cloud">
Cloud instructions
  </Tab>
  <Tab title="OSS">
OSS instructions
  </Tab>
</Tabs>

Checklist

[X] This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.
[X] This pull request references any related issue by including "closes <link to issue>"
[ ] ~If this pull request adds new functionality, it includes unit tests that cover the changes~
[ ] ~If this pull request removes docs files, it includes redirect settings in mint.json.~
[ ] ~If this pull request adds functions or classes, it includes helpful docstrings.~

Preview

https://prefect-bd373955-add_oss_to_the_quickstart.mintlify.app/3.0rc/get-started/quickstart

PrefectHQ / prefect