docs[patch]: Make Docusaurus and Vercel add trailing slashes when navigating by default (#20014)

Should hopefully avoid weird broken link edge cases.

Relative links now trip up the Docusaurus broken link checker, so this
PR also removes them.

Also snuck in a small addition about asyncio

docs/vercel.json

@@ -1,4 +1,5 @@
 {
+  "trailingSlash": true,
   "redirects": [
     {
       "source": "/docs/integrations/providers/optimum_intel",