Rename actions unit to repo.actions and add docs for it (#23733) (#23881)

Backport #23733 by @wolfogre

I neglected that the `NameKey` of `Unit` is not only for translation,
but also configuration. So it should be `repo.actions` to maintain
consistency.

## ⚠️ BREAKING ⚠️

If users already use `actions.actions` in `DISABLED_REPO_UNITS` or
`DEFAULT_REPO_UNITS`, it will be treated as an invalid unit key.

Co-authored-by: Jason Song <i@wolfogre.com>

models/unit/unit.go

@@ -312,7 +312,7 @@ var (
 
 	UnitActions = Unit{
 		TypeActions,
-		"actions.actions",
+		"repo.actions",
 		"/actions",
 		"actions.unit.desc",
 		7,