Add definition of “co-located grids” to the glossary

[MGomezN]

Addition requested in #138

Relevant issues/PRs:

Fixes #138

[leouieda]

Thanks for adding this @MGomezN ❤️

I have a small suggestion for rewording and linking this back to the tutorial.

[leouieda]

Pulled this commit into #139 since they make more sense as a single change.

[leouieda]

Oops, just noticed that I posted messages to the wrong PRs...

[leouieda]

@MGomezN this PR and #140 are better as a single change (adding to the glossary and then using the glossary term in the docs). So I pulled the commit from that PR into this one. So I'll be closing that one and commenting here instead.

In the future, you can keep pushing to the same branch to update the PR if there are further changes you want to make before the PR is merged. The balance of what goes into a single PR and what should be multiple is tricky and we've been playing it by ear. A good rule of thumb is: does this PR make sense on its own? If not, then maybe further changes can be added there. Otherwise, then it would be better as a follow-up PR.

None of this is too strict and it's always possible to change as we go along (pulling commits into separate PRs is doable with a little git magic 😉). So don't worry too much about getting it right from the start. We rarely do and often have discussions along these lines.

[leouieda]

@MGomezN left a few suggestions which I'll merge into this PR so we can get the PR merged as well 🙂 Sorry for the delay on this.