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

[kesara]

This change adds the created datetime in the UTC time zone using ISO
datetime format for HTML output of an RFC.
Example:

  <meta content="2023-09-22T11:16Z" name="created">

Fixes #1032

[cabo]

As per the discussions in SEDATE, +00:00 should be Z now...
(Unless you want to express the document was created in Reykjavik.)

[kesara]

As per the discussions in SEDATE, +00:00 should be Z now... (Unless you want to express the document was created in Reykjavik.)

Python standard library is still not supporting Z [1] but I'll change the code so that xml2rfc adds Z instead of +00:00.

[1] python/cpython#90772

[kesara]

Updated created timestamp to show hours and minutes with Z instead of +00:00.
Example:

<meta content="2023-09-22T11:16Z" name="created">