Group WHATS_NEW entries by date into one section per day

The changelog had grown to 204 repeated `## What's new (DATE) — TITLE`
H2 headings, many sharing the same date. Collapse each date into a single
`## What's new (DATE)` section with every feature demoted to a `### TITLE`
sub-section, so a day's releases read as one dated group instead of a wall
of identical date headers. Body text is unchanged; older untitled summary
blocks stay intact (the one that shares a consolidated date becomes an
"Additional updates" sub-section).

Author: JE-Chen