Consolidate duplicate CHANGELOG files (root vs docs/)

Problem

There are currently two CHANGELOG.md files maintained separately:

• CHANGELOG.md (project root) — used by PyPI / GitHub releases
• docs/CHANGELOG.md — rendered in the documentation site

These are manually kept in sync, which is error-prone. For example, the root file was missing the 0.8.3 entry and the unreleased content added in the docs overhaul branch until it was added manually as part of the v0.8.4 release prep.

Proposal

Pick a single source of truth and either:

1. Symlink docs/CHANGELOG.md → CHANGELOG.md, or
2. Configure the docs build to reference the root file (e.g. via a mkdocs/zensical nav pointer or a docs/CHANGELOG.md that just --8<---includes the root file).

Notes

• Do this in a dedicated PR so the history is clean.
• Ensure the docs site still renders the changelog correctly after the change.