Add `robots.txt` to test sites

[adamschwartz]

As stated in the README, we deploy projects to both a testing site (cloudflare-docs.workers.dev) and a production site (workers.cloudflare.com):

cloudflare-docs/README.md

Lines 48 to 58 in b00f495

	Test sites deploy to:
	```txt
	https://[pathPrefix].cloudflare-docs.workers.dev/[pathPrefix]/
	```
	Production sites deploy to:
	```txt
	https://developers.cloudflare.com/[pathPrefix]/
	```

We should instruct search engines to ignore the test sites via a robots.txt with:

User-agent: * Disallow: /