feat: fill out 3.0 release notes

[itinerare]

I think I covered all the bases, but this could really use:

• Proofreading to make sure it's all coherent, that things are in the right sections, etc
• Checking that nothing's missing that shouldn't be
• Checking if the order and organization of it all makes sense, particularly wrt the "code maintenance" section of the highlights

Mind that these are the release notes, not the upgrade guide. The intent of this is purely to describe the contents of 3.0 without getting into what actually needs to be done to upgrade or engage with the new features. It's a broad overview aimed at answering the question of what's actually in 3.0 in a (relatively) concise and approachable way.

Partly completes #2.

[SpeedyD]

Looks good, I have not confirmed if this is all info or if the info is correct, but the text is clear and concise.

And my apologies for missing a link in the history. =_=;;.. I'll give it another once-over at a later date, to see if perhaps I missed something else.

[itinerare]

No worries! I only spotted the one, so I figured I'd fix it while I was at it (especially since I had to grab the URL anyway).

[perappu]

Fixed a typo, minor additions, swapped -- to proper emdash (—)

[SpeedyD]

Same comment as before. I like how legible it is, clear and concise as we're used to from ol' @itinerare. Please merge it in! :)

[itinerare]

Deeply amusing given my tendency to get verbose in other contexts. Nonetheless, much appreciated!