Fix proxy config in Gitlab Pages nginx examples

What does this MR do?

Adds the proxy_cache off; directive to the example NGINX config for GitLab Pages

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

The default proxy_cache_key leads to pages being cached across GitLab Pages hosts. We weren't expecting NGINX to do any caching, so disabling it is the best option.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-pages/issues/73