Manage CHANGELOG.rst with chloggen

[ocelotl]

Closes #28.

Adopt chloggen, the OpenTelemetry changelog tool, so changes are recorded as individual .chloggen/*.yaml entry files and compiled into a dated version section at release time instead of hand-editing CHANGELOG.rst.

Approach

CHANGELOG.rst stays reStructuredText — the whole repo is rst. chloggen's renderer is format-agnostic via its summary_template config key, so this ships a custom rst template rather than converting the changelog to Markdown. Released history is left byte-for-byte unchanged.

Changes

• .chloggen/config.yaml, TEMPLATE.yaml, summary.tmpl — chloggen config plus a custom reStructuredText summary template (rst section underlines, ``component`` inline literals, anonymous `#N <url>`__ issue links, indent-based subtext). Single changelog → CHANGELOG.rst; components unrestricted.
• CHANGELOG.rst — adds the <!-- next version --> insertion marker (written as an rst comment) and an intro describing the workflow.
• CONTRIBUTING.rst — new "Changelog" section: create an entry by copying TEMPLATE.yaml or via chloggen new, field reference, and how to validate locally.
• RELEASING.rst — the release step now runs chloggen update --version "<version> - <date>" (which consumes the entry files) instead of hand-editing.
• .github/workflows/ci.yml — a new changelog job runs chloggen validate on every PR (via setup-go + go run …@v0.30.0; no Go dependency is added to the Python project).

Verification

• chloggen validate passes on the clean setup.
• A real chloggen update renders correct rst, and the resulting CHANGELOG.rst parses clean through docutils at the default warning level.

Notes

• The changelog was all released history with no hand-written "Unreleased" section, so no lossy conversion was needed.
• CI validates entries but does not force every PR to include one; adding an entry for user-facing changes is documented as a convention in CONTRIBUTING.rst.