Document reproducible builds and check automatically via check-staged-release

With SLING-9307 and SLING-8951 all builds relying Parent 39 or newer should be reproducible. Still some parts are missing:

1. buildinfo files should be uploaded along with the artifacts (https://maven.apache.org/guides/mini/guide-reproducible-builds.html) which allows to easily reproduce build (as they contain info about linebreaks and major JDK versions).
2. The check-staged-release.sh should be updated accordingly to optionally try rebuilding and comparing with the given checksums.
3. https://sling.apache.org/documentation/development/release-management.html should be extended with how to properly check a release after a vote.
4. There should be some documentation covering how end-users could reproduce builds.
5. On the downloads page (https://sling.apache.org/downloads.cgi) there should be a link to the buildinfo files for new/reproducible releases pointing to downloads.apache.org.