Add non wagtail URLs to sitemap.xml

[kesara]

#135 added sitemap.xml to the wagtail website. This only contains the URLs that are generated by the wagtail website. We should expose other URLs that are on www.ietf.org that is not generated by wagtail. Examples:

• https://www.ietf.org/id/
• https://www.ietf.org/archive/id/ (be careful - that one will exhaust your browser's resources if you just open it)
• https://www.ietf.org/ietf-ftp/