Rename actions unit to `repo.actions` and add docs for it (#23733)
I neglected that the `NameKey` of `Unit` is not only for translation,
but also configuration. So it should be `repo.actions` to maintain
consistency.
## :warning: BREAKING :warning:
If users already use `actions.actions` in `DISABLED_REPO_UNITS` or
`DEFAULT_REPO_UNITS`, it will be treated as an invalid unit key.