Update glossary with better definitions and links to episode or manual

[bencomp]

Some items in the glossary are uninformative, such as the items referring to file name extensions. The first sentence appears to be a partially copied sentence from one of the episodes that serves no additional purpose.

Definitions could also link to parts of the tutorial that explain the term, or to the OpenRefine manual that covers the topic.

[bencomp]

I realise that I should include a list of terms to update or include in the issue to make it a good first issue.