rename /buy-in to /why

I've always felt that the title "buy in" of https://reproducible-builds.org/docs/buy-in/ was a bit hard to understand, and now with the new starting page I think so even more.

On the new startpage we address Tech CEOs / Project Owners and the link to the buy-in page is labeled "Find out more about the high-level benefits of Reproducible Builds".

So I would propose to rename the URL "/buy-in" to "/why" (and keep a redirect for /buy-in) and rename the page title from "Buy-in" to "Why Reproducible builds matter".

What do you think?