Avoid "Last updated [timestamp]" footer on each docs page

Avoid "Last updated [timestamp]" footer on each docs page

Set Asciidoctor option 'nofooter' to avoid the annoying timestamps. It
makes updating the website difficult, because always there are "changes"
in each file. Diffing real changes in generated files for the website
before committing becomes difficult. Furthermore, the user does not have
any immediate benefit for most documents, as they change infrequently or
never (e.g. old release notes).

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>