Fix proxy config in Gitlab Pages nginx examples

Nick Thomas requested to merge (removed):fix-gitlab-pages-cache-config into master

What does this MR do?

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

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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.

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