Update guide.md: Reverse Proxying Server-Side Requests

[soupluvr]

Description

Minor guide.md Addition: Added necessary documentation to redirect server-side requests to an external proxy by setting the environment variable(s) HTTPS_PROXY and/or HTTP_PROXY. Included a link to official Coder documentation.

Issue Link

As this is a very minor fix, I did not create an issue.

Fixes

In the using your own proxy section of guide.md, there is no current documentation about forwarding server-side requests to a reverse proxy, which is often required to make the webapp functional behind your own reverse proxy. This is was my first time reverse proxying a web app, and had trouble finding this documentation until I found it in the FAQ Section of the Coder Docs, which I link to in the documentation for further reading.

This is my first PR, apologies for any mistakes!

[code-asher]

Thank you for the PR! And sorry it took so long for a review.

This is probably me misunderstanding, but this http_proxy configuration is meant for the app, not the reverse proxy itself, right? Since this section is about configuring a reverse proxy to access an application from the browser, not how to make an application able to access the internet through a proxy, I wonder if there is a better place we can put it, or if we make a new section.

Maybe we could change ## Accessing web services to ## Accessing web services from the browser and then add a new section called ## Proxy requests made by your application? Or something like that.