feat(html): Add created datetime as a meta tag

This change adds the created datetime in the UTC time zone using ISO
datetime format for HTML output of an RFC.
Example:
```
<meta content="2023-09-21T10:47:04.440964+00:00" name="created">
```

Fixes ietf-tools#1032

xml2rfc/writers/html.py

@@ -2,6 +2,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals, print_function, division
 
+import datetime
 import lxml
 import os
 import re
@@ -473,6 +474,11 @@ def render_rfc(self, h, x):
 
         generator = "%s %s" % (xml2rfc.NAME, xml2rfc.__version__)
         add.meta(head, None, name='generator', content=generator)
+
+    #    o  created - date created in UTC timezone using ISO format
+        current_time = datetime.datetime.now(datetime.timezone.utc)
+        created = current_time.isoformat(timespec="minutes").replace('+00:00', 'Z')
+        add.meta(head, None, name='created', content=created)
 
     #    o  keywords - comma-separated <keyword>s from the XML source