* feat: add ghost prop for collapse
* doc: version of collapse's ghost prop
* refactor: make ghost collapse's less code to a nested style
* chore: remove redundant codes in ghost collapse's less & doc
* doc: add a background wrapper for ghost collapse demo
* doc: dark-theme wrapper bg-color for ghost collapse demo
* test: update snapshot of ghost collapse
* doc: use softer bg-color on ghost collapse demo
* doc: remove disabled panel in ghost collapse demo
