Revamp README and docs for the Query Engine era

[C0staTin]

Summary

Brings the repo's front door in line with the new MindsDB/MindsHub reality and the new docs at https://mindsdb.github.io/engine.

README.md — full rewrite, developer-first

• Plain-text header ("Semantic search over all your data — entirely in SQL"), accurate badges, nav line: Docs · Website · Discord · Contact
• "Where this fits" note: the Query Engine is a standalone open-source project that optionally pairs with MindsHub agents; links the MindsHub vs MindsDB story
• Technical "How it works" (wire protocols, federated planner, knowledge bases, hybrid retrieval, jobs) with an ASCII architecture diagram
• "From zero to semantic search": 6-step SQL walkthrough mirroring the docs syntax, with deep links to docs anchors and the semantic-search explainer video
• Removed: old header image, demo gif, DeepWiki badge, tutorials (dead blog links), Slack, contributor rewards, contributors graph (pulled from the repurposed mindsdb/mindsdb repo), agents examples, all docs.mindsdb.com/mindsdb.com links

docs/index.html — cleanup + SEO

• Removed the MindsDB Cloud section and sidebar entry (cloud.mindsdb.com is unreachable)
• Title sharpened to "MindsDB Query Engine — SQL Reference"
• Added meta description, canonical, robots, theme-color, Open Graph + Twitter card tags, JSON-LD; kept the Google site-verification tag
• New docs/og-image.png (1200×630) matching the docs theme; og:image points at https://mindsdb.github.io/engine/og-image.png

Community surfaces

• CONTRIBUTING.md: issues link → this repo, Slack/newsletter removed, Discord added, Code of Conduct → local file
• Issue templates: contact link → Discord; deleted the ended integrations-contest template

Notes for reviewers

• The og:image/canonical URLs take effect once this lands and GitHub Pages redeploys
• docs/dark_mode.html intentionally untouched (pending a team check on whether anyone uses it)

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