docs: update graphile-llm README with standard header

[pyramation]

Summary

Updates the graphile-llm README to match the standard header format used by sibling packages (graphile-search, graphile-postgis, graphile-connection-filter). Adds centered logo, CI/license/version badges, table of contents, installation section, plugins table, full configuration example, RAG query examples, and license section.

No code changes — docs only.

Review & Testing Checklist for Human

• Verify the npm version badge URL path (graphile%2Fgraphile-llm%2Fpackage.json) resolves correctly once the package is published. It will show a broken badge until first npm publish.
• Confirm the npm install graphile-llm installation instruction is appropriate — the package is currently a workspace package and may not be on npm yet. Consider whether it should say pnpm add graphile-llm or note that it's a workspace dependency.

Notes

• The README now documents the RAG plugin features (ragQuery, embedText, enableRag toggle, defaultChatCompleter) that were added in PR feat: add graphile-llm plugin — embedding, RAG, and text-to-vector for PostGraphile #1008. These features are new and the documentation reflects the current API surface.

Link to Devin session: https://app.devin.ai/sessions/720693f565cc4b8fb4b198ddf75814cc
Requested by: @pyramation

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