Update docs version selector and urls

[discdiver]

Without multiple v3 docs versions, a selector that displays the minor format is no longer strictly true and could be confusing as the current version is 3.1 rather than 3.0.

This PR updates the version selector options and URLS to displayv3 and v2.

Includes redirects.

Static analysis will fail on this PR because the script to generate the Prefect Server REST API docs has changed. This code passed manually after running twice. So after this PR is merged, future static analysis tests should pass for this item.

After:

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
• [ ] 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.