Update Quickstart install instructions for 3.0

[discdiver]

User would get 2.19.x docs if they followed the existing instructions on that page.

Example

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• If this pull request adds new functionality, it includes unit tests that cover the changes
• This pull request includes a label categorizing the change e.g. maintenance, fix, feature, enhancement, docs.

For documentation changes:

• This pull request includes redirect settings in mint.json for files that are removed or renamed.

For new functions or classes in the Python SDK:

• This pull request includes helpful docstrings.
• If a new Python file was added, this pull request contains a stub page in the Python SDK docs and an entry in docs/mint.json navigation.

[billpalombi]

One typo.

I slightly prefer "unreleased code" to "unreleased version" since the code hasn't been assigned a version yet, but I don't feel strongly about that.

[discdiver]

Cool. Updated.