docs: remove duplicate usage link and restore [^4] footnote reference in README

[ameyypawar]

What

Two small README.md fixes (docs-only, no functional change):

1. Removed a duplicate usage link. The "For more usage, please read:" list contained Quantization Types twice — identical text and URL:

   - [Quantization Types](https://docs.vectorchord.ai/vectorchord/usage/quantization-types.html)
   - [Graph Index](https://docs.vectorchord.ai/vectorchord/usage/graph-index.html)
   - [Quantization Types](https://docs.vectorchord.ai/vectorchord/usage/quantization-types.html)   <-- removed
   

2. Restored the orphaned [^4] footnote. [^4] is defined but never referenced inline, so its blog/doc links don't render. Every other footnote ([^1], [^2], [^3], [^5]) has an inline marker, so this adds [^4] to the Accelerated Index Build bullet — on the "hierarchical K-means" clause, matching the footnote's own #hierarchical-k-means anchors:

   ...Powered by hierarchical K-means[^4] and highly optimized disk operations...
   

Happy to relocate the [^4] marker (e.g. onto the "20 minutes" claim instead) if you prefer a different anchor.

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