Secure Development on Pantheon Doc Update

Re: [Secure Development on Pantheon](https://docs.pantheon.io//guides/secure-development/secure-integration)

Priority: Low/Medium/High (choose one, remove the other options)

## Issue Description:

this link isn't working anymore. https://github.com/pantheon-systems/soip-example

## Suggested Resolution

replace it with a working github public gist.