3rd party SSL/TLS certs on TurnKey: convert CER/P7B to PEM

Stuart recently asked via support how to use third party .cer or .p7b SSL/TLS certificates with TurnKey v14.x.

As I don't run any permanent websites, I'm not super familiar with different certificate formats. My only experience really has been through my years with TurnKey and I've only ever encountered the text file .pem certs. So I did a quick bit of research to help Stuart out. I figured that seeing as it's been a little while since I wrote a blog post and this info may be useful for others, I wrote it up. :)