Moved changelog site nav into the header

Repo: KevinJump/changelog

Duplicated the footer site nav (changes / releases / versions) into the header, on its own line above the title, so it's visible without scrolling and doesn't crowd the title/search row
Fixed a CSS source-order bug where .site-nav's justify-content: center was overriding .site-nav-top's flex-end, leaving the header nav centered instead of right-aligned