[Docs] Clarify how NGINX Plugin works in docs (#1653)

## Summary

* Docs now describe how the nginx.template and nginx.conf files interact
* Added comments to nginx.conf + nginx.template so users know they have
to modify nginx.template
* Updated example

## How was it tested?

Build devbox locally, then made sure the nginx example works

Author: Lagoja

docs/app/docs/devbox_examples/servers/nginx.md

@@ -31,10 +31,13 @@ Use `devbox services start|stop nginx` to start and stop the NGINX service in th
 The following helper files will be created in your project directory:
 
 * devbox.d/nginx/nginx.conf
+* devbox.d/nginx/nginx.template
 * devbox.d/nginx/fastcgi.conf
 * devbox.d/web/index.html
 
-Note that by default, NGINX is configured with `./devbox.d/web` as the root directory. To change this, you should modify `./devbox.d/nginx/nginx.conf`
+Devbox uses [envsubst](https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html) to generate `nginx.conf` from the `nginx.template` file every time Devbox starts a shell, service, or script. This allows you to create an NGINX config using environment variables by modifying `nginx.template`. To edit your NGINX configuration, you should modify the `nginx.template` file. 
+
+Note that by default, NGINX is configured with `./devbox.d/web` as the root directory. To change this, you should modify `./devbox.d/nginx/nginx.template`
 
 ### Environment Variables
 ```bash

plugins/nginx/nginx.conf

@@ -1,3 +1,6 @@
+# The nginx.conf in this folder is automatically generated from nginx.template
+# To modify your NGINX config, edit the nginx.template file
+
 events {}
 http{
 server {

plugins/nginx/nginx.template

@@ -1,3 +1,6 @@
+# The nginx.conf in this folder is automatically generated from nginx.template
+# To modify your NGINX config, edit the nginx.template file
+
 events {}
 http{
 server {