Improve sidebar navigation, TOC structure, and doc cleanup

[jswir]

Summary

• Sidebar defaults to collapsed — all sections start collapsed, with only the active page's ancestor chain auto-expanded. State persists within a browser session (sessionStorage) and resets on tab close.
• Restructure Language Reference — group 30+ flat items into subsections: Core Concepts, Data & Types, Expressions & Functions, Aggregation & Analysis, Composition
• Add 13 missing pages to TOC — pages that were linked from content but missing from sidebar nav (Quickstart, Time Ranges, Timestamp Operations, Timezones, Null Handling, Ungrouped Aggregates, Apply Operator, Filter Expressions, Custom Functions, Evaluation Spaces, SQL Blocks, HyperLogLog, API/JSON Results)
• Restyle nested sidebar sections — replace heavy gray background boxes with left-border + indent
• Make active item distinct from hover state (darker background + bold)
• Fix section ID generation — replace(" ", "_") only replaced first space, breaking sessionStorage persistence
• Remove empty window.malloynb — feature moved to GA as calculations_windows
• Fix broken link in experiments.malloynb pointing to deleted window page
• Add experimental flag callout to sql_expressions docs

Test plan

• Verify sidebar starts collapsed on fresh session (clear sessionStorage)
• Verify clicking a page auto-expands its ancestor sections
• Verify manually expanded sections stay open across page navigations
• Verify closing tab and reopening resets sidebar to collapsed
• Verify all new TOC entries render and link correctly
• Verify /documentation/experiments/sql_expressions renders correctly
• Verify no build warnings or broken links (npm run build)