add field 'revdate' to asciidoc files of documentation

add field 'revdate' to asciidoc files of documentation

For reproducibility we need to set up a date that's not depending on the
current time and timezone. We add a extra asciidoc control field
'revdate' and expand the variable behind that while building the package
with the date from the changelog file.

Gbp-Pq: Topic reproducibility
Gbp-Pq: Name add-field-revdate-to-asciidoc-files-of-documentation.patch