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/minio_test.go

@@ -6,6 +6,8 @@ package storage
 import (
 	"os"
 	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
 )
 
 func TestMinioStorageIterator(t *testing.T) {
@@ -13,11 +15,13 @@ func TestMinioStorageIterator(t *testing.T) {
 		t.Skip("minioStorage not present outside of CI")
 		return
 	}
-	testStorageIterator(t, string(MinioStorageType), MinioStorageConfig{
-		Endpoint:        "127.0.0.1:9000",
-		AccessKeyID:     "123456",
-		SecretAccessKey: "12345678",
-		Bucket:          "gitea",
-		Location:        "us-east-1",
+	testStorageIterator(t, setting.MinioStorageType, &setting.Storage{
+		MinioConfig: setting.MinioStorageConfig{
+			Endpoint:        "127.0.0.1:9000",
+			AccessKeyID:     "123456",
+			SecretAccessKey: "12345678",
+			Bucket:          "gitea",
+			Location:        "us-east-1",
+		},
 	})
 }