Remove references to changelog and delete file

Removed all references to `CHANGELOG.md` in documentation and templates, and deleted the `docs/changelog.md` file. This change reflects a shift away from maintaining a changelog in favor of other documentation practices.

.github/PULL_REQUEST_TEMPLATE.md

@@ -88,7 +88,6 @@ assignees: ''
 <!-- Mark documentation changes -->
 
 - [ ] README.md updated (if needed)
-- [ ] CHANGELOG.md updated (for user-facing changes)
 - [ ] Code comments added for complex logic
 - [ ] Docstrings updated for modified functions/classes