fix(seo): prevent contributors.txt from indexing and crawling

[caugner]

Summary

(MP-1094)

Problem

We link to our /contributors.txt without indicating to search engines that these files shouldn't be crawled or indexed.

Solution

1. Mark them with rel="nofollow".
2. Return an X-Robots-Tag header.
3. Disallow /*/contributors.txt via robots.txt.

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How did you test this change?

1. Ran yarn && yarn dev and verified the HTML of http://localhost:3000/en-US/docs/Web/HTML#related_topics
2. Ran cd cloud-function && npm i && npm start and verified the HTTP headers of http://localhost:5100/en-US/docs/Web/HTML/contributors.txt

[argl]

LGTM