Rework Changelog (#531)

* Use consistent header names:
    - "Breaking Changes"
    - "New Error Codes"
    - "Removed Error Codes"
    - "Changed Error Codes"
    - "Bugfixes"
    - "Other Changes"
* Turn section headers into proper markdown headers
* Make sure all changes have a header
* Remove "Introduce" from error codes in the "New error codes" sections
* Turn most entries into proper sentences with proper capitalization and
  punctuation
* Mention that flake-pyi uses CalVer
* Change "Change Log" to "Changelog"
* Fix version number of 25.5.0

Author: srittau