Generate a Changelog file

[honzajavorek]

As of now, we use Semantic Release to generate changelog to GitHub Releases. I'd like to have a tool, which serializes GitHub Releases to a single Markdown/reStructuredText file, so it could become a part of generated documentation. Why?

1. Backup of the changelog (file versus GitHub SaaS)
2. Sometimes when upgrading, it's easier to scan through and follow the changes if they're all listed on a single page. GitHub UI for releases is click-heavy.

The generated file wouldn't get committed to the repository, rather it would be a result of compilation of the docs.

Note: The internet is full of tools to generate GitHub Releases from something, but I'm yet to find a tool to generate a file from GitHub Releases.

Bonus: When we have this, we can generate similar files for Gavel.js and dredd-transactions.

[honzajavorek]

This could be done as Sphinx extension now.