Docs should show latest release by default

[ethanwhite]

Our docs are currently showing the docs from main. When we change functionality this will cause issues for most users until the functionality is released (see #745).

We should change the docs to show the last release by default (not sure if this needs #743 to get merged so we can start actively releasing on GitHub) and then have a dropdown to let folks switch to latest to see the current dev docs. We may also want to include previous releases. Here's an example of the dropdown from sourmash:

We have this same pop up, but only build latest