Misc embedded documentation fixes and enhancements

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• Migrations have been added if applicable
• Significant changes of this merge request have been added to the changelog.

I generally try to make 1 MR per commit, but these are all minor tweaks to the embedded documentation, and having many MRs seemed annoying.

I found most of these by running cargo deadlinks and there are still more to go. I wouldn't mind getting this reviewed and merged before I find and fix all of them though, as that is likely to be a long grind and it can be iterative instead.