refactor: remove old single-page docs renderer

Delete the monolithic +page.svelte that used marked for runtime markdown rendering. Replaced by mdsvex-based per-page routing.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

Author: fullstackjam