docs: clarify 0.9 metrics status and downloads

[fmassot]

Summary

• identify the Parquet metrics and DataFusion work as prototype-only and absent from official v0.9.0 binaries and Docker images
• add the same clarification to the 0.9 upgrade guide
• update the versioned installation page to the exact v0.9.0 release assets

Validation

• verified all four archive names against the published v0.9.0 release assets
• ran yarn clear && yarn build in the quickwit.io repository; the Docusaurus production build passed
• checked the generated 0.9 installation and upgrade pages for the corrected downloads and warning

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Correct the claim that binaries omit the metrics pipeline

The v0.9.0 Linux and macOS archive workflows build with release-feature-vendored-set and release-macos-feature-vendored-set, respectively, and both feature sets explicitly enable quickwit-indexing/metrics; that feature wires matching indexes to the Parquet metrics pipeline. DataFusion and the Docker image may be excluded, but stating that all of the listed work is absent from the v0.9.0 binaries gives downloaded-binary users inaccurate capability information.

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