Improve Sitemap.xml

[neos-bot]

Jira issue originally created by user @kitsunet:

The Sitemap.xml could contain more information and we should make use of all features:

Last modification:
http://www.sitemaps.org/protocol.html

Alternative language:
https://support.google.com/webmasters/answer/2620865?hl=en

Image/Video/News sitemap:
https://support.google.com/webmasters/answer/183668?hl=en&ref_topic=4581190

Jira-URL: https://jira.neos.io/browse/NEOS-1751

[kdambekalns]

The last modification is included since 41d9cd9, but it only considers the document node itself. Changes to the content are not reflected… see #40.

Alternate languages can be provided, but using the sitemap and meta tags would be useless.

Image, video & news are rather specialised use-cases, I'd say those can be implemented in separate packages.