docs: introduction / marketing website — landing + guided onboarding

[alirezat66]

Summary

Ship a polished introduction path for flex_track: positioning, “why not raw SDK calls”, a 2-minute quickstart, architecture overview, and clear CTAs to deep docs + the flagship example.

Problem

• The package is powerful but dense; new users need a guided first experience beyond README walls-of-text.

Goals

• A landing experience (homepage or equivalent) with strong next-step navigation.
• Visuals where helpful (diagrams, short GIFs), with accessibility basics.

Acceptance criteria

• Landing page exists with: value proposition, key features, “who it’s for”, and primary CTAs (Docs / Example / pub.dev / GitHub).
• Guided onboarding page (or section) with a minimal end-to-end path (setup → track → optional widgets).
• Navigation is coherent: landing ↔ docs ↔ example instructions.
• Accessibility: meaningful headings, image alt text, readable contrast on default theme.
• Repo README links prominently to the public site “Start here”.
• Release note mentions the site launch when applicable.

Notes / links

• If the site source is not currently tracked in git, this issue should include decision + migration (version website/ in-repo vs separate repo) as part of acceptance.

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Content checklist (optional)

- [ ] Inspector GIF / screenshot - [ ] “Compared to alternatives” short section (honest, high-signal) - [ ] FAQ: consent, sampling, Flutter Web inspector limitations