Packaging polish: source_ref, homepage_url, CHANGELOG

[stuartjohnpage]

Background

Small hex/docs hygiene items found in mix.exs:

• docs() has no source_ref, so "View Source" links on hexdocs point at the default branch instead of the release tag. Fix: source_ref: "v#{@version}".
• homepage_url points at the org (https://github.com/revelrylabs) rather than the repo.
• There is no CHANGELOG.md. RELEASES.md documents the release process, not the history; Hex users look for a changelog. Add one (Keep a Changelog format), backfill recent releases from git history/release notes, wire it into the docs extras, and add "update CHANGELOG" to the RELEASES.md checklist.

Acceptance Criteria

Scenario: Source links target the release

• Given a published version on hexdocs
• Then "View Source" links resolve to the matching v* tag

Scenario: Changelog exists and stays current

• CHANGELOG.md exists, covers releases through the current version, appears in hexdocs extras
• RELEASES.md preparation checklist includes updating it