docs: polish README with original branding and real animated demos

[danishxsethi]

Summary

Refresh the project's front door for both users and contributors:

• Add original graphite-and-amber branding and four newly captured, genuine product-demo GIFs, with static alternatives; the complete visual set is approximately 0.69 MB.
• Make Docker/Python onboarding, authentication, API response shapes, SDK/CLI/MCP choices, and contributor entry points clear.
• Distinguish source-only doctor, --output, and per-request user_agent support from the published v0.1.2 packages.
• Add isolated, reproducible native capture/render tooling with fail-closed output checks and graceful server/browser cleanup.
• Correct the existing example.com API journey assertion to check extracted body text instead of a heading no longer present in the current upstream page's Markdown. This fixes the CI failure without changing extraction behavior.

Related issue: N/A.

Type

• Bug fix
• Feature
• Refactor
• Docs
• CI/tests

Checklist

• Tests added/updated and pytest passes
• ruff check blazecrawl_core is clean
• Node surfaces: cd sdks/node && npm test passes (if touched)
• If this touches network/ egress/SSRF: a failing-without-fix security test is included
• Public API changes are documented in README/docs
• No new mandatory cloud dependency introduced

All six GitHub CI jobs passed on commit c7250bd: Python lint/tests/build, Python SDK, Node SDK/MCP, Python MCP, Docker quickstart smoke, and security. The network-specific checklist item is not applicable because no network/egress implementation changed.

Local verification also covered the actual Python/Node SDK and Bash/PowerShell HTTP snippets, fresh CLI/map/crawl/MCP/URL-rejection captures, media-tooling lint/format checks, links and cross-file anchors, published-wheel feature compatibility, asset budgets, and light/dark/narrow rendered previews. The corrected API journey test passes locally. No product runtime/API/SDK/network code changed; the media scripts perform their own runnable output checks. Docker was unavailable locally, but the GitHub Docker smoke job passed.

Security impact

No changes to the product's egress, SSRF, or authentication surfaces. The demo instance binds to loopback with authentication enabled and a new temporary key that is never displayed or persisted in captures. Pillow is scoped to optional media generation, not core runtime dependencies.