seo: tighten hub title + unify descriptions (human-voiced)

[dineshbyte]

Unifies the three descriptions (meta/og/twitter) onto one human-voiced line and drops the brochure phrasing; the twitter description also still said 'EPUBs' (removed feature) — now corrected.

What & why

Type of change

• Content — new lesson / track, or reworked existing content
• Content fix — typo, clarity, accuracy, or diagram fix
• Bug fix — broken link, layout, search, etc.
• Feature — site/tooling capability
• Chore / docs / CI

Scope

Checklist

• node scripts/check-broken-links.js passes (no broken internal links)
• node scripts/validate-content.js passes (SEO tags, interview data-levels, JSON-LD, sitemap in sync)
• New/renamed pages are added to docs/sitemap.xml
• Each page has a unique <title> (· StackDepth), meta description, canonical, and Open Graph tags

For content changes

• Visual-first: the concept is carried by diagrams/tables/cards; prose only labels them (passes the "skim with text too small to read" ship test)
• Accurate & cited: non-obvious claims cite high-trust sources (RFCs, specs, vendor/standards docs) — no claims from memory
• Interview panel present where expected, with valid data-level values (core / senior / staff / design)
• Quiz options are length-balanced; dark and light modes both legible

Notes for the reviewer