explicitly mention the source revision and language for translations

[mverburg]

The DE and EN versions are the languages maintained by the owners of this repository.

When bumping a version it can happen that community supported translations (ES, IT and NL at the moment of writing) are based on an earlier version.

It would be good to explicity mention what source language and revision of the template where used for the translation in order to understand how up-to-date the translations are.

[gernotstarke]

add

Template Version {revnumber}. {revremark}, {revdate}

to about-arc42.adoc and define these variables at the beginning of arc42-template.adoc

That leads to a duplicate mention of the version in html, but is ok for other formats, afaik.