Make the NILMTK start page task-first

[nipunbatra]

What changes

• replaces the promotional hero with four direct choices: data, models, comparable results, or names/schema
• makes the data/model/benchmark choices open the matching install tab
• rewrites section headings and copy as short answers: repository choice, install, citation, model contribution, and help
• keeps exact Python 3.11 + uv, Docker, benchmark, citation, and real-data guidance
• removes unused signal-illustration and hero CSS after the markup change (net fewer lines)
• checks every tab/deep-link target and rejects the intentionally retired promotional copy
• updates the site workflow to current checkout/setup-python major versions

Verification

• python3 scripts/check_site.py — 20 IDs, 49 links/assets, 7 tab targets, and 9 canonical URLs passed
• Playwright at 1440×1100 and 390×844 — task routes, tab selection, keyboard navigation, mobile menu/Escape behavior, and horizontal overflow passed
• browser console — no errors
• desktop and mobile screenshots inspected; full-page desktop layout inspected
• CSS selector audit — no landing-page class left without HTML/JS usage
• git diff --check — passed

The published install commands were checked against the current core CLI/README, nilmtk-contrib extras and lazy exports, and NILMbench benchmark extra/CLI.