feat: about extendr

[JosiahParry]

we need more copy on "what" extendr is. Right now from here its somewhat clear what extendr is, but not super.

Either an About or a section further down on this page that describes in a more "marketing" way what extendr is.

[kbvernon]

does the new landing page and introducton/get-started fix this? or do we need something else?

[kbvernon]

ok, i think extendr.rs/introduction/ kinda addresses this, but right now it's got some issues:

1. it's too focused on r and rextendr and doesn't really give a sense of what is happening on the rust side - you know, where the magic happens.
2. it also doesn't say enough about the overarching structure of these development tools, how they all hang together.
3. we should probably add links to extendr rust docs and rextendr r docs here, too

i believe we should steal some content from the extendr readme and add it here, at least a simple example of what lib.rs looks like.