docs: update changelog with CI, Pages, and CONTRIBUTING entries

Author: jcardozo-eth

CHANGELOG.md

@@ -26,6 +26,13 @@ based on the [nestor schema](https://nbn-resolving.org/urn:nbn:de:0008-202305031
 - GitHub Actions CI workflow (`.github/workflows/validate.yml`)
   running `nix develop -c just validate` on push and pull request
 - Versioning strategy documented in README following eCH-0018 namespace conventions
+- `CONTRIBUTING.md` with language conventions, dev setup, branching, conventional
+  commits, versioning strategy, release process, and eCH-0018 compliance rules
+- `CHANGELOG.md` following [Keep a Changelog](https://keepachangelog.com/) format
+- GitHub Actions release workflow (`.github/workflows/release.yml`) triggered on
+  `v*` tags: creates GitHub release, attaches XSD, builds and deploys GitHub Pages
+- Jekyll-based GitHub Pages site with auto-generated landing pages per major version
+- `.gitignore` for generated content and IDE files
 
 ### Changed (relative to nestor schema)