Removes 8.x RNs and What's new from main

[benironside]

Fixes #6041 as part of #257

8.x release notes, as well as what's new, need to be removed from main in preparation for 9.0.

[github-actions]

A documentation preview will be available soon.

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[mergify]

This pull request does not have a backport label. Could you fix it @benironside? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[nastasha-solomon]

Thanks for doing this @benironside! The local preview looked good and didn't contain any leftover What's new files/pages or 8.x release notes. Approving early so it's ready to merge once the CI checks pass.

[nastasha-solomon]

@benironside you might need to remove this if you re-add the what's new page.

[benironside]

run docs-build

[nastasha-solomon]

Thanks, everything looks good @benironside! It's interesting that two chapter tags (namely the new one added here) don't affect the CI checks or TOC structure at all. I don't know enough about the book tags to suggest a change though so if it works, I don't want to mess with it anymore. Anywho, looks good to merge!