docs: remove architecture section and expand notable differences
Remove the architecture/ pages (pytree, implements, gsobject, drawing)
and key-concepts page. Consolidate their most useful content into
notable-differences.md, which now thoroughly covers immutability, array
views, RNG, PyTree registration, control flow/tracing, profile
restrictions, numerical precision, and the @implements decorator.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>