Broken links on download page

There are lots of broken links on the download page.

These are mainly because early releases did not have separate sha256 hashes.

For example: apache-calcite-1.13.0-src.tar.gz.sha256 is missing; should be apache-calcite-1.13.0-src.tar.gz.mds

Wrong file name: apache-optiq-0.9.0-incubating-src should be apache-optiq-0.9.0-incubating-source-release

Could not find any instances of calcite 0.8 or optiq 0.8
Was that release before the ASF?

Please fix the links and test them.