Changelog site: dated archive, collapsible months, entries reorg

Repo: KevinJump/changelog

Homepage now shows only the last 5 dates with entries, with a link to a new /archive/ section
Added /archive/ (year index) and /archive/YYYY/ (per-year pages), grouping entries by month — months are collapsible via native <details> — then by date
Reorganized src/entries/ into YYYY/MM/ subfolders to keep the source tree browsable as it grows; public entry URLs unchanged
Moved the archive/about nav links from the header into a second centered row in the footer