docs: create sections for basic behavior of labelme at labelme.io/docs

e.g.,

<img width="937" height="310" alt="Image" src="https://github.com/user-attachments/assets/0f28f04a-1d36-4b65-a2db-544cadeb08e3" />

https://github.com/wkentaro/labelme/issues/642#issuecomment-3328150767