add usage example

[Young-Flash]

I noticed the user issue on discord, I think we can create a example dir on the codebase, including comprehensive usage example for different bindings, showing all the prerequisites and dependencies, making users who not familiar with opendal run the project with less confuse.

[Xuanwo]

Nice idea! I think we can move examples to top level with enough docs so that users can start more smoothly.

[Xuanwo]

Will be covered by #2457