website: consider redirect / routing

In removing the `/current/` default path in PostGraphile docs (and build, and maybe Grafast docs) we end up with a blank page when visiting `postgraphile.org/postgraphile/` or similar "root" level docs pages. I found this plugin which might make things easier: https://docusaurus.io/docs/api/plugins/@docusaurus/plugin-client-redirects#redirects but it also has warnings at the top to investigate server side redirects first. We could just make sure there are no broken links, but the question remains what if someone found themselves on that page?