sg: add a docs changelog command

Created by: jhchabran

It's not easy to follow changes within the docs and a the output of git log is not good enough to deduct what happened exactly.

What if we instead had:

$ sg docs changelog
012345: fix foo bar
- added a/b/c.md
- updated d/e.md
- deleted z.md
- renamed xx.md
012345: another fix
(...)