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

openapi-first/sitemap.xml

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+</urlset>