fix(site): center docs content on wide screens (#378)

Starlight right-aligns the content against the TOC (margin-inline: auto 0)
on pages with both a sidebar and a TOC, dumping all the empty space on the
left edge — very lopsided on wide monitors. Center the content in its pane.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>