Show docs for latest release by default

[Hasnep]

This would replace the current default which is showing the docs for the main branch. Copied from an idea in Zulip by @Anton-4.

[Anton-4]

Repeating my reply from roc-lang/roc#5718 for record keeping:

There's been a bit of mixup, on zulip I was talking about "Should we display the code for the latest release when going to https://github.com/roc-lang/basic-cli instead of the main branch". I think we should keep the docs site https://www.roc-lang.org/packages/basic-cli pointed to the main branch to prevent issues when the URL is shared, for example when someone thinks that they are sharing the docs for 0.4.0 but the content changes over time.

[Anton-4]

We could change the URL to https://www.roc-lang.org/packages/basic-cli/branch/main and also add https://www.roc-lang.org/packages/basic-cli/latest-release