Prevent search engines to index v1-* domains of our site

For example, see this search result:

Google is recommending results from v1-7.docs.dapr.io and even v1-1.docs.dapr.io. We should add a `robots.txt` file to non-prod sites to prevent search engines to index them.

Alternatively, we should add a [canonical URL](https://developers.google.com/search/docs/advanced/crawling/consolidate-duplicate-urls) that always points to the prod page, so search engines know that the content in v1-* is duplicate.