Site overhaul

Our documentation is woefully inadequate and has been for some time. As part of an effort to remedy this, we are doing a substantial overhaul of this website. 

Note: this effort is supported by an R Consortium [ISC grant](https://r-consortium.org/all-projects/2025-group-2.html#rextendr-community-focused-development-for-scientific-computing-with-rust-and-r). 