doc: Add field 'revdate' to asciidoc files

doc: Add field 'revdate' to asciidoc files

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.

Note!
Unfortunately using the modified files in the way it's originally
intended breaks the build of the source by using dpkg-buildpackage twice
in a row. Until this issue is solved the added fields are now commented
out.

Forwarded: Not-Needed

Gbp-Pq: Topic reproducibility
Gbp-Pq: Name doc-Add-field-revdate-to-asciidoc-files.patch