Document the requirement for PKCS#8 keys, and fix generate-cert.sh script

[ameba23]

See https://github.com/flashbots/attested-tls-proxy/pull/74/files#r2635941834

• The requirement that only PKCS#8 formatted keys are accepted should be documented.
• ./scripts/generate-cert.sh should be fixed to always produce PKCS#8 keys regardless of openssl version
• Fix related misleading commend in ./src/main.rs