Define aligning, broadcasting, merging, concatenating, combining in new glossary

[gwgundersen]

Over at #3352, we're working on a new page describing key Xarray terminology for #2410. Since it's already a big PR, and we may need to iterate on the definitions a few times, this issue is for extending the glossary once that PR is merged. Some terms are to add are: aligning, broadcasting, merging, concatenating, combining.

[TomNicholas]

Happy to help with this!