Improve and publish the cuML changelog #8521

Improve the cuML changelog so users can understand the notable changes in each release from the repository, GitHub Releases, and the versioned cuML documentation.

The existing CHANGELOG.md is largely generated from merged PRs, includes changes that are not relevant to users, and has fallen out of sync with GitHub Releases. Release notes are also not available through the regular documentation at https://docs.nvidia.com/cuml.

Desired outcome

• Treat CHANGELOG.md as the canonical source for cuML release notes.
• Write release notes for cuML users rather than as a comprehensive list of merged PRs.
• Begin each release with a short Highlights section.
• Organize entries under the standard Added, Changed, Deprecated, Removed, Fixed, and Security categories.
• Describe the user impact and migration path for breaking changes.
• Include relevant issue or PR links and contributor attribution.
• Exclude CI-only changes, test maintenance, forward merges, dependency housekeeping, and internal refactors unless they have a user-visible impact.
• Finalize the changelog at the beginning of code freeze.
• Publish the same curated release section as the corresponding GitHub Release description.
• Publish the complete changelog as a top-level Release notes page in the regular cuML documentation. Each versioned documentation build should contain the history available through that release.
• Document cuML's deprecation policies in the user documentation and link to them from the changelog where applicable.

Synchronizing GitHub Release descriptions should preferably be automated. If automation is impractical, then we should simply synchronize the GitHub release notes as part of our regular release process.

As part of the initial cleanup, we should backfill the changelog from at least 26.06 onward and consider backfilling earlier releases where practical.