WEB: Add robots.txt to disallow docs preview
With the new doc previewer system, we have multiple copies of our web and docs in the `/preview/` path of our domain. I initially added a `robots.txt` file in that directory to be excluded by crawlers, but I double-checked, and seems like crawlers just check for the file in the root. So, it needs to be added here.
Thanks @datapythonista 