adding proxy configuration #20231107 #20230503 in 4314 issue

[AntonEliatra]

Description

Adding details on proxy configuration for cross cluster search and basic details for proxy configuration itself using Nginx

Issues Resolved

#20231107 - /docs/latest/security/authentication-backends/proxy/ | Even though everything is set, I am still facing 401 error #20230503 - /docs/latest/security/access-control/cross-cluster-search/ |What about other details? What if my port is proxied?

Checklist

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[AntonEliatra]

Thanks @scrawfor99 I tested this fully, using 2 separate clusters prior to raising the PR, therefore can confirm this does indeed works as described

[hdhalter]

Thanks, @scrawfor99. Let's have someone from the cluster team take a look. I'll ping @rohiwali.

[hdhalter]

Hi @natebower , when we come across a proper noun that is getting flagged by Vale, do we always add it to our approved list or handle some other way? This is the name getting flagged: https://www.nginx.com/. Thanks!

Update: I just checked our style guide and this is already included as an accepted term, just capitalized.

[AntonEliatra]

Thank you @ankitkala, that's been updated now

[hdhalter]

Thanks for your thorough review, @ankitkala ! I'll push this through to doc review.