rfc: Add RFC 8508 for IMAP REPLACE capability

Author: stephanbosch

docs/core/summaries/rfc.md

@@ -102,6 +102,7 @@ This is a non-exhaustive list of RFCs that Dovecot implements/supports.
 - IMAP Extension for STATUS=SIZE ([[rfc,8438]])
 - IMAP “$Important” Keyword and “\\Important” Special-Use Attribute
   ([[rfc,8457]])
+- IMAP REPLACE Extension ([[rfc,8508]])
 - Internet Message Access Protocol (IMAP) - SAVEDATE Extension ([[rfc,8514]])
 - IMAP4 Extension: Message Preview Generation ([[rfc,8970]])
 - IMAP Response Code for Command Progress Notification ([[rfc,9585]])