openssl: add subcommand pages (#4886)

pages/common/openssl-x509.md

@@ -0,0 +1,20 @@
+# openssl x509
+
+> OpenSSL command to manage X.509 certificates.
+> More information: <https://www.openssl.org/docs/manmaster/man1/openssl-x509.html>.
+
+- Display certificate information:
+
+`openssl x509 -in {{filename.crt}} -noout -text`
+
+- Display a certificate's expiration date:
+
+`openssl x509 -enddate -noout -in {{filename.pem}}`
+
+- Convert a certificate between binary DER encoding and textual PEM encoding:
+
+`openssl x509 -inform {{der}} -outform {{pem}} -in {{original_certificate_file}} -out {{converted_certificate_file}}`
+
+- Store a certificate's public key in a file:
+
+`openssl x509 -in {{certificate_file}} -noout -pubkey -out {{output_file}}`