When generating certificates from LetsEncrypt we get 4 `.pem` files. These are:
1. fullchain.pem
2. privkey.pem
3. cert.pem
4. chain.pem
What are they all?
privkey.pem: private key
cert.pem: public key
chain.pem: certificate chain
fullchain.pem concatenation of cert.pem and chain.pem
https://community.letsencrypt.org/t/what-are-those-pem-files/18402