Add definition of glue to docs

[rowanbarrie]

👓 What did you see?

No formal definition of "glue" is present in the Cucumber docs

✅ What did you expect to see?

A brief definition of "glue" somewhere before related flags and properties (e.g. --glue, cucumber.glue) are introduced.

For a newcomer it is not necessarily clear what glue is, and why it's needed. It is stated in passing in step-organization.md and implied several places in api.md, but a simple, brief definition somewhere before related code is introduced would make it easier to understand.

[mlvandijk]

Hi @rowanbarrie - thanks for pointing that out.
Is this something you'd be willing to contribute? The place for it imho would be under "terms": https://github.com/cucumber/docs/tree/main/content/docs/terms
We should add a file called glue.md, similarly to user-story.md but with content for "glue" instead of "user story".