docs: reproducibly build docs by using deterministic timestamp
docs: reproducibly build docs by using deterministic timestamp

Use the SOURCE_DATE_EPOCH timestamp [1] if it is available for
timestamps in the tarballs and the HTML documentation.

Also fix permissions inside tarballs to normalize umask
variations.

[1]: https://reproducible-builds.org/specs/source-date-epoch/

f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver