[DOC] Migrate other resources of the mxGraph documentation #615
Description
Other resources
Non exhaustive list, to be done when the mxGraph content has been integrated.
### Tasks
- [x] Remove CSS and JS. They were only needed by the mxGraph documentation. They are kept during integration if some special rendering/behavior must be ported to the maxGraph documentation. #621
- [x] Decide what to do with remaining images. Create specific content to use them or simply drop them --> remove. #642
- [x] Integrate information from [old-mxgraph/api-specification.txt](https://github.com/maxGraph/maxGraph/blob/6e6009aebd1eb5b32cddb3aa49c946d748287fdc/packages/website/old-mxgraph/api-specification.txt) into the developer documentation. #642