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>

routers/install/install.go

@@ -116,7 +116,7 @@ func Install(ctx *context.Context) {
 	// Application general settings
 	form.AppName = setting.AppName
 	form.RepoRootPath = setting.RepoRootPath
-	form.LFSRootPath = setting.LFS.Path
+	form.LFSRootPath = setting.LFS.Storage.Path
 
 	// Note(unknown): it's hard for Windows users change a running user,
 	// 	so just use current one if config says default.