export and respect SOURCE_DATE_EPOCH

export and respect SOURCE_DATE_EPOCH

to improve reproducibility of pve-docs, pve-doc-generator and
pve-doc-mediawiki builds, as well as man pages generated by
pve-doc-generator when called via dpkg-buildpackage.

timestamps embedded in the documentation now always reflect the last changelog
entry of the package to which the documentation belongs (pve-docs for html,
epub and pdf, individual packages for man pages).

ported from pve-docs:b965ad63ad80b5967f5bc1f0327eb548b6049045

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>