6.9 release notes

[pdurbin]

What this PR does / why we need it:

Which issue(s) this PR closes:

• Closes Release 6.9: Create release notes #11879

Special notes for your reviewer:

Here's a preview: https://github.com/IQSS/dataverse/blob/11879-6.9-release-notes/doc/release-notes/6.9-release-notes.md

Suggestions on how to test this:

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Is there a release notes update needed for this change?:

Additional documentation:

[pdurbin]

@poikilotherm thanks for the detailed review and all the suggestions. I implemented the vast majority of them but left a couple unresolved. Please see the replies I left.

Also: please please very much please use one sentence per line. It makes typing (and reading) suggestions so much easier. Thanks.

I need more space than I have here to unpack this so I created a topic in Zulip about it: https://dataverse.zulipchat.com/#narrow/channel/446770-docs/topic/one.20sentence.20per.20line/near/563628045

@landreev thanks for your suggestions as well. I incorporated them.

@jggautier @qqmyers thanks for making fixes.

From my perspective, we can probably merge this PR at this point. I think it's the only thing holding us back from a release. Plus, we can always fix it in post. (We can edit the live release notes after release, I mean.) Let's talk about it more at standup on Monday.

Thanks, all! ❤️

[coveralls]

coverage: 24.233%. remained the same
when pulling f4292e6 on 11879-6.9-release-notes
into fd75558 on develop.

Most changes were implemented. We don't want the merge button to be blocked. Oliver and I plan to discuss other proposed changes tomorrow.