[DOCS] Create glossary entry for Drupal "contexts"

[ysuarez]

• If applicable, link to the documentation page relevant to your issue below.
  https://islandora.github.io/documentation/user-documentation/glossary/

• If this is a request for new documentation, where in the existing documentation does this documentation belong?

• Describe your documentation request.

In Islandora 2 a lot of work done through Drupal "Contexts" (info here, and since this is a common English word that may be used in talking about systems, I would like to add it to the glossary to make it more clear how it may be used. Though not sure yet what the definition should be yet.

https://www.drupal.org/project/context

• Which version of islandora are you using?

Islandora 8 and above

• What user roles is this documentation intended for?

Collections Manager, Developer, Systems Administrator

• What implementations of islandora does this documentation request apply to?

all implementations

Any related open or closed issues to this bug report?

Screenshots: