Add section about variance and positions

[eernstg]

Resolves #5254.

Staged: https://dart-dev--pr6105-glossary-variance-sep24-fxrxoasd.web.app/resources/glossary#variance-and-variance-positions

[dart-github-bot]

Visit the preview URL for this PR (updated for commit 3dfa993):

https://dart-dev--pr6105-glossary-variance-sep24-fxrxoasd.web.app

[parlough]

Sorry about the delay in reviewing this, I hope to get to it soon! Thanks again for working on this :D

[parlough]

Thanks so much for working on this @eernstg! I'm not very familiar with these concepts, so I naturally read it from a perspective of someone brand new to these terms. As a result, my questions and suggestions might be incorrect, so please feel free to push back on anything. At the very least, I hope my confusions and potential misunderstandings can at least help you identify where readers might go astray.

I left a few other small comments for the second pass so I can validate my understanding before leaving misleading suggestions. :)

Thanks again and sorry about the delay in reviewing this! I'm super excited to have something tangible in the docs to reference for these terms, and I can see it helping others as it was already insightful to me.

[eernstg]

Made several changes based on the review.

[parlough]

Thanks so much for the answers to my questions and your edits @eernstg! This is looking great :D

I have a few follow-up comments for you to consider, but nothing blocking. I'm excited for us to have this useful reference!

[eernstg]

That's much better, thanks!

[eernstg]

@parlough, I think this could be landed now. WDYT?