docs: overhaul README with tagline, How it Works, Ecosystem, and more

[torinkwok]

Summary

This PR delivers a comprehensive README overhaul for the open-source launch of Knowhere:

• Header: Added centered tagline (Prepare unstructured data for AI Agents), pyramid badge layout (Python, Stars, Build, Community, Container, License), and text nav links (Website, Docs, Self-Host, Dashboard).
• Project description: Replaced generic copy with a precise one-liner positioning Knowhere as an open-source data layer for AI Agents and RAG.
• Cloud CTA: Added a [!NOTE] alert directing users to knowhereto.ai with a $5 free credits offer for new sign-ups.
• News section: Two entries — open-source launch (May 7, 2026) and version 2026.04.30.1 release (Apr 30, 2026).
• How it Works: Added a full three-stage pipeline explanation (Parsing, Memory Graph, Agentic Retrieval + Vector-based RAG) with a Mermaid diagram showing parallel retrieval paths.
• Ecosystem table: Listed all related repositories (dashboard, self-hosted, Python SDK, Node SDK) with emojis in both README.md and CONTRIBUTING.md.
• Supported Formats: Checklist-style section with ✅ Supported and ⏳ Coming Soon groups, plus a contribution invite.
• Citation: Added ## Citation section with a BibTeX @software entry and a CITATION.cff file for GitHub's native "Cite this repository" button.
• Communication & Contribution: Replaced Project Governance with friendlier Communication and Contribution sections referencing Discussions, Issues, and good first issues.
• We're Hiring: Added a closing section with a base64-obfuscated contact email.

Test plan

• Review rendered README on GitHub after merge.
• Confirm Mermaid diagram renders correctly.
• Confirm CITATION.cff triggers the "Cite this repository" button on the repo sidebar.
• Confirm all badge links are functional.

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