Update nginx configuration documentation

[nekoprog]

Questions	Answers
Branch?	9.x
Description?	Add fastcgi_param to forward HTTPS to local server. This is useful for Cloudflare Tunnel user and SSL-enabled reverse proxy to prevent too many redirections loop and mixed content served browser errors and allow user to Enable SSL everywhere in back office.
Fixed ticket?	#27177 #39657 #39190 #35751
Sponsor company	Your company or customer's name goes here (if applicable).

And these from PS forums for example
https://www.prestashop.com/forums/topic/1094161-ssl-certificate-mixed-content-on-the-site/
https://www.prestashop.com/forums/topic/1087769-prestashop-8-err_too_many_redirects/
https://www.prestashop.com/forums/topic/1037143-insecure-mixed-content-with-ssl-1541/#google_vignette

While many reported issue occurs on Apache, this PR will fix if user is using Nginx.

[kpodemski]

thank you @nekoprog

[nekoprog]

@kpodemski Sorry I messed up by using tabs instead of spaces, and skip deployment with hugo before submitting PR.
So here's the updated version #2098 .