Fix all possible setting error related storages and added some tests (#23911)

Follow up #22405

Fix #20703 

This PR rewrites storage configuration read sequences with some breaks
and tests. It becomes more strict than before and also fixed some
inherit problems.

- Move storage's MinioConfig struct into setting, so after the
configuration loading, the values will be stored into the struct but not
still on some section.
- All storages configurations should be stored on one section,
configuration items cannot be overrided by multiple sections. The
prioioty of configuration is `[attachment]` > `[storage.attachments]` |
`[storage.customized]` > `[storage]` > `default`
- For extra override configuration items, currently are `SERVE_DIRECT`,
`MINIO_BASE_PATH`, `MINIO_BUCKET`, which could be configured in another
section. The prioioty of the override configuration is `[attachment]` >
`[storage.attachments]` > `default`.
- Add more tests for storages configurations.
- Update the storage documentations.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/storage/local_test.go

@@ -8,6 +8,8 @@ import (
 	"path/filepath"
 	"testing"
 
+	"code.gitea.io/gitea/modules/setting"
+
 	"github.com/stretchr/testify/assert"
 )
 
@@ -55,5 +57,5 @@ func TestBuildLocalPath(t *testing.T) {
 
 func TestLocalStorageIterator(t *testing.T) {
 	dir := filepath.Join(os.TempDir(), "TestLocalStorageIteratorTestDir")
-	testStorageIterator(t, string(LocalStorageType), LocalStorageConfig{Path: dir})
+	testStorageIterator(t, setting.LocalStorageType, &setting.Storage{Path: dir})
 }