Add a simple glossary page to top nav with basic definitions.

[lauralorenz]

This PR is to add the new page to nav and one-liner definitions of things already defined in the website, not add any new definitions yet.

I think this provides another useful, short-form variation of this information for the website audience, as well as is intended to facilitate the goals in capturing variations in terminology discussed in comment kubernetes/community#8210 (comment).

Quick link to netlify preview page in question: https://deploy-preview-41--kubernetes-sig-multicluster.netlify.app/references/glossary/

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lauralorenz

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[lauralorenz]

/sig multicluster

[lauralorenz]

Triage notes:

• ClusterProperty case in particular is still modern terminology according to other people in the community. Will update the definition to not reference it as an old name but inline regarding the Kind name in the definition of the About API.
• Do old names belong on the Glossary??? Or just in terminology.md??? Or in a different page??? Laura's bais is to handle that in a separate PR so she is cleaning this one up of any of this.

[lauralorenz]

Updated with changes from triage notes, ready for review again.

[RainbowMango]

/lgtm
I see all comments have been addressed, ready to go.
Thank you, @lauralorenz, for doing this!