fix: set the API document site to the primary URL of package

[eitsupi]

Related to #700

This is required for automatic linking support via downlit, as downlit expects the very first URL in the URL field to be a pkgdown site.

downlit::autolink("polars::as_polars_df()")
#> [1] "<a href='https://rpolars.github.io/man/as_polars_df.html'>polars::as_polars_df()</a>"

^{Created on 2024-01-28 with reprex v2.0.2}