Add MarkDown changelog and use it by default

[felixfontein]

SUMMARY

Uses ansible-community/antsibull-changelog#139 to render the changelog both as RST and MarkDown. References the MarkDown version from README since GitHub stopped rendering the RST version.

ISSUE TYPE

• Bugfix Pull Request
• Docs Pull Request

COMPONENT NAME

changelog

[felixfontein]

Result: https://github.com/felixfontein/community.crypto/blob/changelog/CHANGELOG.md