sieve: Add examples

[cmouse]

JIRA: DOV-8101

[sirainen]

How about "default Message-ID header based tracking.

[sirainen]

Lets use some example that makes more sense. ChatGPT suggests:

require ["duplicate", "imap4flags"];

# If the subject starts with "ALERT:", check for duplicates based on the full subject
if header :matches "Subject" "ALERT: *" {
    if duplicate :seconds 60 :header "Subject" {
        setflag "\\seen";
    }
}

[sirainen]

This example's usage of variables extension is rather unnecessary, since it could be directly matching the subject. ChatGPT suggests:

require ["variables", "header", "fileinto"];

# 1. Use wildcards to capture the tag inside brackets
# Example: "[Support] Help needed" matches "Support" into ${1}
if header :matches "Subject" "[*] *" {
    
    # 2. Normalize the tag to lowercase
    set :lower "tag" "${1}";

    # 3. Use an explicit list to ensure you only file into existing folders
    if string :is "${tag}" ["urgent", "receipts", "travel"] {
        fileinto "Folders/${tag}";
    }
}