Make ln-invoice's CHANGELOG & RELEASING look like Nostrino's

Also change the RELEASING.md step #3 to read

3. Update `CHANGELOG.md` with changes since the last release. Follow the existing `CHANGELOG.md` format, which is
derived from [this guide](https://keepachangelog.com/en/1.0.0/)

Compare the project's CHANGELOG.md and RELEASING.md with Nostrino's corresponding files, starting with the existing release documentation and step #3. Done means the CHANGELOG and RELEASING formats match Nostrino's conventions and step #3 contains the requested wording and Keep a Changelog reference.

Written by the indexing model from the issue text.