CHANGELOG.md

https://keepachangelog.com/en/0.3.0/

• Added: For any new features that have been added since the last version was released.
• Changed: To note any changes to the software’s existing functionality.
• Deprecated: To note any features that were once stable but are no longer and have thus been removed.
• Fixed: Any bugs or errors that have been fixed should be so noted.
• Removed: This notes any features that have been deleted and removed from the software.
• Security: This acts as an invitation to users who want to upgrade and avoid any software vulnerabilities.

CONTRIBUTING.md

https://git.sr.ht/~seirdy/moac/tree/master/CONTRIBUTING.md

ARCHITECTURE.md

https://matklad.github.io//2021/02/06/ARCHITECTURE.md.html
For smaller projects it probably makes more sense to put the architecture as a section in CONTRIBUTING.md instead.