Bevy 0.19 release note pass (#24176)

The release is nigh! One of the most important tasks is to get the
release notes ready.

I've done them all in a single batch to avoid endless spam and to be
able to consider them in context.

Two of them are deliberately omitted: the BSN notes ( @cart has claimed
these) and render-graph-as-systems ( for @tychedelia).

Key things I focused on when editing and writing these release notes:

- what is worth teaching about, and how much detail does it need
- how do we contextualize and motivate each feature
- is the usage clearly demonstrated without taking up too much space
- the tone should be fun, clear, and optimistic throughout

---------

Co-authored-by: Trashtalk217 <trashtalk217@gmail.com>
Co-authored-by: Adam__ <67244228+Based-A@users.noreply.github.com>