Refactor MTA-STS policy files: streamline index.html, update mta-sts.txt, and add comprehensive mta-sts.md example file

mta-sts.md

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+# Example: `mta-sts.txt`
+
+This is an example of an **MTA-STS policy file** with all possible values.  
+Save this file as `.well-known/mta-sts.txt` and serve it over HTTPS at:  
+`https://mta-sts.example/.well-known/mta-sts.txt`
+
+---
+
+## Policy Fields
+
+```plaintext
+version: STSv1
+# The version of the MTA-STS policy. Must be "STSv1".
+
+mode: enforce
+# The policy mode. Possible values:
+#   enforce   - SMTP servers must enforce the policy.
+#   testing   - Policy is in testing; violations are reported but not enforced.
+#   none      - No policy is applied.
+
+mx: mail.example.com
+mx: *.mail.example.net
+# The list of MX patterns (hostnames) that are valid for your domain.
+# Wildcards (*) are allowed only as the left-most label.
+
+max_age: 86400
+# The time (in seconds) that the policy is valid for.
+# Recommended minimum is 86400 (1 day). Maximum is 31557600 (1 year).
+```
+
+---
+
+## Example Full Policy
+
+```plaintext
+version: STSv1
+mode: enforce
+mx: mail.example.com
+mx: *.mail.example.net
+max_age: 86400
+```
+
+---
+
+For more details, see:  
+https://datatracker.ietf.org/doc/html/rfc8461