Define BDD when used in user documentation

[fulldecent]

Some people don't know what that means.

[sukrosono]

Surely, could you participate on https://github.com/chaijs/chai/discussions/1399 ?

If you had cristal clear suggestions to define it, but I would take @lucasfcosta input first.

or it could be that it's already on the documentation elsewhere.

[koddsson]

This is a great suggestion! I think we'd welcome pull requests to better define BDD.

[lucasfcosta]

I would definitely like to have that in the docs. BDD can be a bit tricky to define, but I'd definitely be willing to review.