mail: add page (#2669)

pages/common/mail.md

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+# mail
+
+> The command operates on the user's mailbox if no argument is given.
+> To send an email the message body is built from standard input.
+
+- Send a typed email message. The commandline below continues after pressing Enter key. Input CC email-id (optional) press Enter key. Input message text (can be multi-line). Press "Ctrl-D" key to complete the message text:
+
+`mail --subject={{"subject line"}} {{to_user@example.com}}`
+
+- Send an email that contains file content:
+
+`mail --subject={{"$HOSTNAME filename.txt"}} {{to_user@example.com}} < {{path/to/filename.txt}}`
+
+- Send a tar.gz file as an attachment:
+
+`tar cvzf - {{path/to/directory1 path/to/directory2}} | uuencode {{data.tar.gz}} | mail --subject={{"subject line"}} {{to_user@example.com}}`