Add "Cluster" to the glossary

[mbelton-buildkite]

With clusters becoming generally available in the upcoming release and being an important concept to understand, I think it should have an entry in the glossary. I've taken a first pass from the info on the existing page, but very open to changing it.

[buildkite-docs-bot]

Preview URL: https://2222--bk-docs-preview.netlify.app

[timriley]

This is a good addition!

If I were to suggest one thing, it would be to emphasise that clusters contain queues of agents, since that's the main concept that people end up interacting with when they use clusters. I know you include queues in the latter part of the description, but that feels a bit like it's "burying the lede" IMHO :)

You might have a better idea about how to incorporate this into a short description, but I've provided one suggestion in case it helps :)

Otherwise, I'd still be very happy to see this added as-is. We can finesse it later.

[gilesgas]

Thanks @timriley for reminding me of this PR! 🤦

I was meant to merge it a while back. I'll do so once its build completes successfully