doc: Use towncrier to handle release notes

Towncrier is a utility to produce useful, summarized news files.

See:

• https://pypi.org/project/towncrier/
• https://towncrier.readthedocs.io

Custom configuration for xkbcommon:

• New fragments are located in the changes directory.
• 5 sections:
  • Model: changes/models
  • Layout: changes/layouts
  • Option: changes/options
  • Miscellaneous: changes/misc
  • Build System: changes/build
• 3 news fragments:
  • Breaking changes: .breaking
  • New: .feature
  • Fixes: .bugfix

ChangeLog is renamed to ChangeLog.md because the tool requires .md extension to write in markdown format.

Fixes #452 (closed)