docs: Update NGINX proxy example with best practises

[tomowatt]

We've had a customer raise some concerns about our current example so adding extra information for security as the current example shows loose config in terms of not showing usage of the allowing only from the Buildkite IPs and limiting access to specific URLs for the use with Buildkite

By listing the the specific and only URLs/Endpoints required, we can ensure users follow best practices.

Will update PR for review when confirmed all the URLs/Endpoints required for GitHub Enterprise Server.

[buildkite-docs-bot]

Preview URL: https://2743--bk-docs-preview.netlify.app

[gilesgas]

Hi @tomowatt , thanks very much for this contribution! 🎉

Based on your description, I'm not sure if this is still a work in progress or if it's ready to merge in.

Regardless, I've marked this PR as a draft (to indicate that it's not yet ready to merge), but I've also already approved this PR. Therefore, if you deem this PR as ready to merge in, please click the Ready for review button and the PR should 🤞 be ready to merge in. Please proceed with the merge (since it's now approved), but if you're unable to do this, please let me or @mbelton-buildkite know and we'll do this for you.

I only implemented some minor wording tweaks around the link to the NGINX docs, which I hope are OK with you...

[tomowatt]

Had a second round of testing and checking and happy with the results 🚀