Add experiment routes to robots.txt

What does this MR do?

In gitlab-experiment, we now have a concept of experiment coupled urls. These urls are currently used in markdown to instrument the number of visits a url gets from that markdown content. This is done to avoid having to add html to markdown templates, and this approach can be used in a lot of ways, like in email content and places where we'd otherwise not be able to instrument usage.

Because of this, if these urls appear in public content (public readme or other markdown files), we want to disallow bots from traversing these urls and artificially inflating their visit counts.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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