docs: create shared glossary for terms used in documentation

In general we should have a shared glossary. It would also be best if we were using terms in a consistent manner across documents (ref the glossary we had in https://github.com/IBM/algorithm-nexus/pull/22/changes)

_Originally posted by @AlessandroPomponio in https://github.com/IBM/algorithm-nexus/pull/29#discussion_r3124096229_
            