docs: add missing CHANGELOGs

CHANGELOG.md

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+# GovTool Proposal Pillar Changelog
+
+For changelogs please see individual backend and frontend packages.
+
+- [Backend Changelog](./backend/CHANGELOG.md)
+- [Frontend (pdf-ui) Changelog](./pdf-ui/)

pdf-ui/CHANGELOG.md

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+# @intersect.mbo/pdf-ui Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+As a minor extension, we also keep a semantic version for the `UNRELEASED`
+changes.
+
+## [tag-xxxx](https://github.com/IntersectMBO/govtool-proposal-pillar/releases/tag/xxxx) 2024-xx-xx
+
+### Added
+
+- 
+
+### Fixed
+
+-
+
+### Changed
+
+-
+
+### Removed
+
+-