chore(releasing): use GitHub Release assets for stable downloads

[pront]

Summary

Motivation

Stable Vector artifacts are already uploaded to GitHub Releases, while the website, installer, and release tooling still direct users to packages.timber.io. Before retiring that hosting, users need a clear deprecation notice and an artifact-by-artifact migration plan.

Changes

• Route stable website download buttons and package-manager examples to immutable GitHub Release assets.
• Download stable installer archives and Homebrew checksum inputs from GitHub Releases.
• Add a 0.58.0 deprecation fragment and generated deprecations index. The release page and always-current deprecations index are the policy-prescribed announcement mechanism; they retain existing URLs for the deprecation period and include a user-facing migration table. It also clarifies that Helm chart packages already use GitHub Release assets, while helm.vector.dev remains their repository-index endpoint.
• Keep nightly URLs on packages.timber.io.
• Correct the ARMv7 RPM filename to the published armv7hl asset.

Vector configuration

N/A

How did you test this PR?

• cargo vdev deprecation generate
• cargo vdev deprecation show
• cargo vdev deprecation check
• make -C website cue-build
• make -C website cue-fmt
• make check-markdown
• make check-clippy
• make -C website cargo-data production-build
• bash -n distribution/install.sh
• Verified the v0.57.0 GitHub Release asset inventory and rendered download links.

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Follow-up migration and cleanup plan: Complete packages.timber.io artifact migration and retire obsolete content #26104

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