Add robots.txt

The site currently serves a 404 for /robots.txt (verified live). An
explicit allow-all robots.txt is the baseline crawlers expect from a
project site; files in public/ are copied into the export root, so this
lands at documentdb.io/robots.txt. A Sitemap directive can be added once
sitemap generation lands.

Author: guanzhousongmicrosoft

public/robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Allow: /