Add an example for locking static files behind HTTP Auth in Nginx config documentation page

[shadyvb]

Context

Clients wanting their sites to sit behind HTTP Auth can use PHP Basic Auth config to do so for PHP requests, but they need to set that in Nginx custom config as well for static resources. It'd help if we can provide sample config in the docs to let them know how to do so.

Acceptance criteria

• As a visitor, when navigating to the Nginx custom config page in documentation, I should see a sample config for locking specific paths behind HTTP Auth.

[veselala]

Hey team! Please add your planning poker estimate with ZenHub @mikelittle @robindevitt @shadyvb

[veselala]

@roborourke we've estimated this one to 1 SP and moved it to the spring backlog