Add images for documentation on signal transforms

[doursand]

This PR is around adding images to improve the documentation on signal transforms

[doursand]

@wyli I am creating this pr to add visualisations and to improve the documentation on signal. Could you please review it ?

[wyli]

Could you please fix the typo in the filename continous->continuous

[wyli]

this script also needs to be updated to automatically add the figures the documentation page: https://github.com/Project-MONAI/MONAI/blob/857e92b5645f0a30d955770d54fcb5fa6a3efd7f/monai/transforms/utils_create_transform_ims.py#L463-L466

[doursand]

thx, I was precisely wondering how to add the images to the doc :)