feat: serve docs flat at /<repo>/ with no redirects

- collect.py copies each repo's built site contents directly to a
  top-level <repo>/ dir (was libs|apis|wiki/<repo>/site nesting)
- nginx uses `index index.html lib/index.html api/index.html` so
  /dagpipe/ -> lib/index.html, /auth-server/ -> api/index.html are
  served internally with no redirects
- _index regenerates links against the flat layout
  (/dagpipe/lib/, /auth-server/api/, /mongo-ops/, /blog/...)
- config.yml static entries point at vendored blog/ + media-manager/
- Dockerfile copies per-repo dirs flat into the nginx html root
- removed stale libs/, apis/, wiki/, tutorials/ category trees

nginx.conf

@@ -3,7 +3,7 @@ server {
     server_name _;
 
     root /usr/share/nginx/html;
-    index index.html api/index.html;
+    index index.html lib/index.html api/index.html;
 
     location / {
         try_files $uri $uri/ /index.html;