[Dev][Website] Changelog generation should use commit messages

Currently, archery release changelog generate will get issue titles from JIRA.
This is suboptimal as often the title of a JIRA points to a general problem, but doesn't describe the specific changes. Sometimes even, the JIRA title is misleading as the chosen solution is different as the one envisioned by the submitter.

It would be better to fetch the git commit titles and use the first line as the change summary.